Fair warning, this post is massive:

• I break each of the languages down into several parts
• In every one of those parts I talk about what makes each language comparatively easier or harder than the others

Feel free to skip to the sections you're curious about — or to the end, where I conclude with my personal ranking of the languages.

To people trying to pick a language to learn:

A lot of people get hung up on the supposed “difficulty” of a language. How long the Foreign Service Institute or their favorite YouTuber says it will take to learn. There’s some merit to that: some languages are indeed more complex than others, at least in certain specific ways. If you’re a native English speaker, you’re going to find it easier to adapt to the conventions of some languages and cultures than others. All that will have an impact on whether you ultimately succeed or fail.

I also think these things are a massive distraction.

Whether your source’s “time till fluent” number is (supposedly) 600 hours, 2,200 hours or 10,000 hours — all of those numbers are so big that they’re meaningless. What really matters is the hour ahead of you right now:

• What are you going to do with it?
• Are you excited to do this, or is it something you're forcing yourself to do?
• Is it practical for you to consistently do this thing?

The two languages I’m most confident in are Spanish and Japanese. My primary motivation for learning languages is to read, and I’ve read dozens of books in each one. Common sense should tell you that Japanese was much harder to learn than Spanish was… but that wasn’t true. I was forced to learn Spanish in school, and largely dreaded it. Spanish meant little more than homework. Japanese was something I did on my own volition, and I largely enjoyed it. Because I enjoyed it I was much more consistent about making time to study and generally finding opportunities to use the language.

The inertia behind those hours added up: to reach a level where I was able to read my first book took three years in Japanese... and eight years in Spanish.

So, if you’re reading this post to help you decide which language to learn, stop.

Pick the language whose culture you’re most interested in.

This is going to take you hundreds (more likely thousands) of hours. The only way you’re going to put in those hours is if it’s something that gives you energy, rather than something that takes energy.

Pick your passion (poison?), do your homework, and then work on convincing yourself that you don’t feel guilty about watching 600 hours of k-drama last year because it was, actually and technically, Korean practice.

Who am I to make this comparison?

Please note that I am not a linguist — I am just someone who speaks all three languages, to some degree.

• Japanese — intermediate; I did part of my university in Japan, passed the JLPT N1 (the highest level proficiency test), and have read dozens of books in Japanese.
• Mandarin — intermediate; I have lived in Taiwan for 6 years, have used Mandarin daily since arriving, and have passed the TOCFL 5 (higher than HSK6).
• Korean — beginner; I read manhwa in Korean, but don't plan to learn it to a high level. I do know quite a bit about Korean phonology and sentence structure. Anyway, take my comments here with a grain of salt.

How hard is each language's...

Loosely speaking, I will organize each of these sections in terms of difficulty. The language appearing first is the one I feel is hardest, the language appearing last is the one I feel is easiest.

(Note: I'm a pronunciation nerd, so those sections will be a bit longer than the others.)

Pronunciation (Phonetics)

Korean pronunciation is, in my opinion, the hardest by far.

• Korean has sounds that don't exist in English, it differentiates sounds that English does not differentiate, and it has numerous spelling rules that affect pronunciation.
• Mandarin has its fair share of new and difficult sounds, but the language is very consistent. Learners will succeed after a period of initial struggles.
• Japanese has many little subtleties to master that are largely "cosmetic" in nature — even if ignored, they won't really impede someone's ability to understand you.

Korean phonetics

Stumbling blocks abound for both the mouth and also the ears. Many of Korean's sounds are familiar, but the act of consciously differentiating them will cause problems for learners.

Learners will see a few totally new vowels (like /y/, that "French" u sound, and /ɯ/ — which sounds sort of like a sound you'd make after getting punched in the stomach), but a bigger challenge lies in differentiating three vowels that feel very similar in the mouth: eo, o, and u (/ʌ/, /o/, and /u/, respectively.) O and u are pronounced virtually the same, but the lips protrude (see example) with u and not with o. The sounds o and eo are easy to distinguish, but they often appear in succession, as with the -eoyo verb ending, which creates something of a tongue twister — just try saying the word 무거워요 (mugeowoyo, heavy) five times fast. All four of those vowels are different.

The consonants, unfortunately, are not much easier. Korean stops (sounds like t, d, p, and b) all get distinguished three ways: aspirated, unaspirated, and "tensed." English speakers will be used to making the first two, but not differentiating them — the k in king is aspirated, the one in sky is not.     Tensed consonants involve putting more tension around the glottis (the ~upper area of your throat), and they cause the proceeding vowel to be pronounced with a slightly higher pitch. These three sounds may initially sound identical to learners, but mixing them up will confuse Korean people.

Even after a learner becomes confident with Korean's array of sounds, the battle isn't quite won. The language includes many spelling and pronunciation rules (essential ones / advanced ones), so you can't always expect that a given word will be pronounced as it looks. (To be clear, words are irregular in seemingly consistent ways... it will just take time to learn the rules, and that added complexity will make Korean pronunciation's already-difficult learning curve even steeper.)

For an in-depth of discussion of similar-but-different sounds, see this article.

Mandarin phonetics

A few initial hurdles, but smoother sailing after that.

On top of a few sounds that just don't exist in English, like the vowels /y/ and /ɤ/, Mandarin includes a few groups of consonants that will sound very similar to English ears (such pinyin s/sh/c — /s/, /ʂ/, and /ts'/, respectively). As with the three-way Korean sounds, confusing these three Mandarin sounds will make it difficult for Mandarin-speaking people to understand you.

Sounds aside, pinyin itself may sometimes interfere with your pronunciation. Some of this interference comes in the form of "negative" crossover from English — pinyin b is not actually a /b/ sound but rather an un-aspirated /p/, like the p in spy (as opposed to the p in pie or the b in bye.) Some of that intereference comes from the fact that the pinyin aren't always very clear — pinyin u can refer to both the sound /u/ and the sound /y/, but those two sounds are not interchangeable. Using one when you should have used the other will raise eyebrows. (Note that /y/ is sometimes notated with an umlaut (two dots above the u), but not always. There are also certain spelling rules that will help you figure out how u should be pronounced in a particular word.)

Given the additional difficulty of the tones (discussed below in prosody), learners may need to book a few sessions with a pronunciation tutor to sort these initial difficulties out.

Japanese phonetics

Japanese pronunciation is often described as easy: native speakers will likely understand learners even if they never spend any time at all thinking about pronunciation.

While I wouldn't personally call Japanese phonetics easy, as there really is quite a bit of subtle stuff going on under the surface, the language is merciful to learners in that it doesn't have any sounds that could reasonably be confused. For example, the Japanese a sound is more central than English's (compare Japanese /ä/ to English's /a/), and its sh sound is actually a different sound than English's (compare Japanese /ɕ/ to English's /ʃ/) — but, unlike Mandarin, there is only one thing that sounds like an a and sh sound in Japanese. As such, even if a learner mistakes them, native speakers will understand what the learner was going for.

Learns will eventually notice that some vowels get devoiced in certain situations (the word for shoes, kutsu, sounds sort of like k'tsu), and will likely eventually notice that the character ん (n) actually represents several sounds. These things sound confusing if you read about them on Wikipedia, but they're also fairly intuitive and you'll likely make the adjustments correctly without realizing that you'd even done so.

All in all, though, Japanese learners will likely only really struggle with one thing: Many English vowels are actually dipthongs (say the word no slowly in a mirror and watch what happens to your lips — there's a hidden u sound snuck in at the end there.) Japanese vowels do not become dipthongs. Whereas English no is /noʊ/, Japanese no would just be no.

Pronunciation (Prosody)

I'll go ahead and call Mandarin the hardest because everybody knows that tones are scary, but we could be a bit more nuanced with this if we really wanted to.

• Mandarin has tones, and those will be a major headache. When you start learning Mandarin you will not feel comfortably saying literally anything for quite awhile. However, the tones will eventually click and become second nature.
• Japanese doesn't have tones, but it does have what's called pitch accent. It will take some getting used to. Japanese rhythm also works differently than English's.
• Perhaps making up for its phonetics, Korean prosody seems to be very simple and straightforward. Huzzah.

Mandarin prosody

The tones are hard, but the silver lining here is that the tones are very much a front-loadead source of difficulty. They will be a major headache for a period of time, but will become largely second nature before long.

You should know that we actually use all of Mandarin's tones in English. The difference is that Mandarin uses tones in a mechanical way*, in order to contrast syllables within a word, whereas we use tones in English primarily to show our emotions. (If you're talking with someone, and are about to take a breath, you'll raise your pitch up higher before you take the breath to signal to the listener that you aren't done talking yet.) Let me me bold here: every single person reading this article will eventually master the tones, but the process will feel awkward.

The bigger issue for me, personally, has been related to what I'll call the "perception" of tones: how much of a tone shift is too little/alright/too much? A few years back my Mandarin level became the topic of discussion at a family reunion (I am married to a Taiwanese woman), and I was surprised to hear many people agree that I had a tendency to not differentiate the tones. They all sounded kind of the same. This shocked me because I thought I tended to exaggerate them. Upon hearing this feedback I emphasized my tones to a degree that I thought was surely comical... only to be met with several nods of approvement. Let this be a cautionary tale: how you perceive the tones is merely your perception, and you unfortunately won't be able to tell how what you're doing sounds to native spaekers. You'll need feedback to find the sweet spot with the tones (and, really, all the sounds you learn.)

(* Small side note: Mandarin speakers also adjust their tone of voice to show emotion, just as we do in English. This isn't as confusing in practice as it seems in theory. Mandarin's tones are relative, not absolute — what matters is not a pitch of a syllable itself but rather the difference between one syllable and the next syllable. So long as your pitch is moving in generally the right direction, that's often good enough. As such, to communicate surprise, a speaker can simply shift the pitch of their entire sentence upwards.)

Japanese prosody

Whereas the difficulty of Mandarin's tones are very front-ended, the difficulty of Japanese prosody is more back-ended in nature. It could largely be ignored without causing too much trouble (though you will sound markedly foreign) — but for learners who care about their accent, Japanese demands long-term attention and dedication.

Japanese does not have tones, but it does have what's called pitch accent (ten-minute explanation). In English we use what's called a stress accent. This means, for example, that the pu of computer is stressed: slightly lengthened, made slightly louder, pronounced with a bit more oomph. ComPUter. Japanese doesn't do any of that — it accents syllables with pitch alone. Imagine that you were sitting at a keyboard (online keyboard): press any white key, then go up two white keys. The lower key is for unstressed syllables, the higher one is for stressed syllables. So, for comparison, the Japanese word taberu (to eat) follows the same "pattern" as computer... but rather than getting extra oomph, the syllable be is merely raised in pitch. Using the wrong pitch pattern would sound sort of like saying compuTER instead of comPUter.

Japanese has four total pitch-accent patterns, and like English, the accent of words is largely unpredictable. If it's something you care about, you'll have to memorize the pattern of each new word you learn.

A few other things that might prove tricky for learners:

• English is a stress-timed language, which means that we "reduce" certain vowels to an "uhh" sound (/ə/, called the "scha") to make the word smoother to pronounce. Japanese doesn't do this. All vowels will always be pronounced with their "full" sound (unless devoiced, as mentioned above in the phonetics section.)
• English has a tendency to connect syllables — when we say my name is, we actually say something like m' nay mis. Japanese doesn't do that. Each Japanese syllable (called a mora) gets an equal "beat" of rhythm. As such, the word honda is actually pronounced ho-n-da, with three beats — not hon-da, as we say in English, with two beats.

Korean prosody

So far as I can tell, and according to my teacher from my teacher (and Jun, 2002), standard Seoul Korean prosody can basically be summed up with a three-step process:

1. The first syllable of a sentence begins on a relatively lower pitch
2. The pitch of your voice raises on the second syllable
3. If the sentence is a question, you again raise your voice on the sentence's final syllable; if your sentence is a statement, your voice will remain consistently high throughout

Or, as put in Jun, 2002:

The Intermediate Phrase in English is the domain of downstep, and is delimited by a phrase accent, H- or L-; the Accentual Phrase in standard (Seoul) Korean is demarcated by a phrase-final High tone.

(In other words, when a low pitch comes up in a Korean sentence, it's signalling the start of a new phrase.)

And that's it! It'll take a bit of getting used to, because the pitch of our voices fluctuates quite a bit throughout a sentence in English, and you'll want to do that while speaking Korean... but Korean prosody is still less complex than Mandarin or Japanese.

A few other things to note:

• Unlike English, and basically like Japanese, Korean syllables are each pronounced with a roughly equal "beat" of time — this will take a bit of getting used to.
• Koreans tend to stress the first syllable of a word, not one of the middle syllables (as we often do in English) --> more information in The Sounds of Korean (Choo, 2003).
• Vowels that follow tensed and aspirated stops (see above in the phonology section where I talked about how Korean sounds like b/p/t/d have ae three way contrast) tend to be pronounced with a higher pitch than they would otherwise (Cho, 2017).
• While my Korean teacher swears by the above 3-step process, and it's been very effective for me personally, it seems like Korean intonation is actually a bit more complicated than that (see p3~7 of Kim, 2013).
• Here's a PhD thesis on Korean Prosody, if you're interested in the topic.

Grammar

Korean here beats out Japanese by an ever so slight margin. Both are harder than Mandarin.

• On a big-picture level, Korean and Japanese grammar is remarkably similar. If you take any paired Korean and Japanese sentence, you can line the parts of the sentences up pretty much perfectly. However, Korean particles come in pairs, whereas Japanese ones don't. Several Korean grammatical constructions are also more complex than their Japanese counterparts. As such, I personally feel that Korean grammar is slightly more difficult than Japanese grammar. (My wife, who is a JP>CN translator/interpreter and has passed the TOPIK 5, says it's a tie.)
• Mandarin grammar is quite similar to English grammar in that building sentences largely consists of taking immutable blocks (characters) and putting them in the right order. Mandarin grammar will challenge you (espceially when it comes to making longer/more complex sentences), but unlike Korean or Japanese, you won't have to worry about any conjugations or inflections.

Korean grammar

For a color-coded picture that basically sums up what I'm saying below at a glance, please see the beginning of the Japanese section.

Korean is a subject-object-verb (SOV) language, whereas English is an SVO language: We'd say I ate a pizza in English, but Koreans would say I pizza ate. Then, English is what's called a right-branching language: descriptions of nouns tend to go the right of the noun — the kids who returned from school are now eating. Korean is a left-branching language: all that description must go to the left of a noun — the school-from-returned kids are now eating. This left-branching business can cause headaches because allows things that would be entire sentences/clauses in English to get tossed around as if they were a single simple noun (stone, pizza, dancer, etc.)

As a result, you'll oftentimes find yourself looking at Korean sentence and feeling like you need to find a map in order to figure out what is doing what.

An additional level of difficulty comes from the fact that Korean uses postpositions (particles), rather than prepositions, as we do in English. This is simple in theory: instead of saying at the park you would say something like (the) park-at. Where it gets complex is that Korean requires you to explicitly mark a bunch of things that we don't explicitly mark in English, like subjects and direct objects. Whereas we know pizza is the direct object in a sentence like I eat pizza because it comes after eat (which is a transitive verb), Korean requires you to tack the direct-object-marking particle 을/를 onto the end of pizza. In other words: to correctly speak Korean, you have to be aware of what each part of your sentence is doing so that you can correctly mark it.

Finally, Korean is an agglutinative language — that is to say, Korean verbs (and adjectives) have many, many inflections. (An example of an inflection in English would be when run changes to runs.) In practical terms, this means that whereas English grammar basically requires you to take words and put them in order, Korean requires you to memorize many different forms for each verb (and them put them in order.) In more practical terms:

• English tends to line words up: I go --> I want to go --> I wanted to go
• Korean tends to give words a very long tail: kada (to go) --> kago sipda (want to go) --> kago sip-eossda (wanted to go)

Japanese grammar

A single glance is enough to understand why Japanese grammar is difficult for English speakers: see this picture.

The saving grace is that, precisely because of all these inflections, Japanese (and Korean) grammar ends up feeling quite formulaic. Sentences are very much bound by logic, and unlike English or Mandarin, where a lot hides behind sentence structure, this logic is very transparently visible to you. Once you get a grasp on these languages' you will appreciate it.

Japanese and Korean grammar are quite similar, and the above points all apply equally to Japanese. However, many Japanese grammatical structures aren't quite as complex as their Korean counterparts:

• Aside from the sound N, Japan's writing system consists entirely of blocks of consonant-vowel pairs. Korean's writing system is an alphabet, just like English's, and it has many spelling rules / irregularities. As such, whereas in Japanese you merely learn a fixed pattern and then copy/paste it onto (basically) every verb, Korean requires that you make some small adjustments depending on how a word is spelled. This seems like it should be trivial, but given how complicated Korean and Japanese grammar are, you'll appreciate having one less thing to worry about when learning Japanese.
• Unlike Japanese (again, with the exception of N), Korean syllables are allowed to end in consonants. While Japanese and Korean (largely?) seem to have the exact same particles, the majority of Korean particles actually come in pairs: one is used when a syllable ends in a consonant, the other used when a syllable ends in a vowel. Again, particles are complicated enough, and you'll appreciate not having to bother with this in Japanese.
• Japanese goes into less "detail" with a number of grammatical constructions. For example, in Japanese, verbs can directly modify nouns. You can simply say the was-running man without needing to do anything special to your verb. In Korean, you must first add a certain inflection to verbs if you want to modify a noun with them... and each verb tense (men who run, the running man, the was-running man, the will-be-running man) requires a different inflection.

(Note: I don't think I could pass the TOPIK 2 test. There are many, many Korean grammatical strcutures I haven't learned yet. It may well be the case that certain Korean grammatical structures are simpler than their Japanese counterparts... but so far as beginner grammatical structures go, Korean takes the cake.)

Mandarin grammar

The above two sections were slogs, so I'm happy to be able to say that Mandarin grammar is not nearly as complex. You have a bunch of immutable blocks (the characters), and just like English, all you have to do is put them in the right order. For lack of better words, compared to Japanese and Korean, Mandarin grammar felt very intuitive.

That isn't to say that Mandarin grammar is easy, just that it isn't as complex: it has many, many, many less moving parts to worry about. Even so, there are still things that will trip you up:

• Mandarin is very regular, but the "prototypical" Mandarin sentence is different than in English. (Here are a few examples of common Mandarin sentence structures.) The structure that really stands out to me is that of time duration, the formula for which is Subject + Time "when" + Location + Verb + Object + (duplicate) Verb + Time "duration." To say I read a book at the library for four hours this afternoon, you'd say I this afternoon at the library read a book read for four hours.
• In English, grammatical aspect is baked into our verb tenses. You don't need to think about it beyond picking the right verb tense. Mandarin doesn't have verb tenses, but it does use the character 了 to communicate the perfective aspect, and it will take time for you to get comfortable "manually" manipulating the perfective aspect in your sentences.
• Small changes in sentence structure can significantly change the meaning of a sentence in ways that often aren't apparent unless you have already learned the particular structures in question. As such, you will quite often find yourself in situations where you know every charcater in a sentence but don't quite understand what the sentence is communicating. For example, both 我在台灣住了四年 (I + in Taiwan + live + perfective particle + four years) and 我在台灣住四年了 (I + in taiwan + live + four years + perfective particle) mean I've lived in Taiwan for four years, but the first one sounds more like you're not currently living in Taiwan (i.e., you're talking about a past experience) and the latter makes it seem like you're still in Taiwan (i.e., from four years ago until now).
• Mandarin has a lot of separable verb+noun pairs that are sort of reminiscent of English's phrasal verbs (verb + preposition, i.e. get up or call out). These verb-noun pairs give you options: you can either leave them as a unit or you can separate them and sandwhich a bunch of stuff between them. For example, to say that you read a book (看書) for four hours, you can either separate the verb-noun pair and say 我看四個小時的書 (I read + 4 hours + book) or you can duplicate the verb and say 我看書看了四個小時 (I + read + book + read + perfective particle + 4 hours). People use these forms interchangably, so you'll have to be aware of both.

Writing system and vocabulary

I'm going to go ahead and rate Japanese as being more difficult than Mandarin here. I think I have good reason for that, but I may be biased, as I went into my Mandarin studies already knowing a few thousand characters.

• Japanese requires one to learn many Chinese characters, and virtually all of the characters have multiple ways they could be pronounced.  You also have to juggle three writing systems at once.
• Mandarin requires one to learn more characters than Japanese does, but most characters only have one reading. This streamlines things much more than you might expect.
• Korean's Hangul is a brilliantly efficient writing system. There are many spelling rules to learn, but those are minor headaches, rather than brick walls.

I've lumped vocabulary into this section, but I'm only going mention it here as a universal point of difficulty. Unlike learning Spanish, where English and Spanish's shared Latinate roots mean you go into your studies already "knowing" several thousand words, you're on your own with Mandarin, Japanese, or Korean. You'll notice some common loanwords like coffee (kōhī in Japanese, kafei in Mandarin, keopi in Korean), but there aren't a ton of them and it's not possible to guess when a loanword does or doesn't exist.  

Japanese writing system

If you've got a moment, please watch How Japan Overloaded Chinese Chracters (6m) by NativLang. I'm rating Japanese as being more difficult than Mandarin because, with Japanese, you have to deal with all of the difficulties of the Chinese characters but only get to enjoy some of the advantages. The whole system is just... awkward. It's like having a pair of great shoes that are a few sizes too big — the quality is there, but they'll often trip you up.

The first thing to get out of the way here is that Japanese juggles three writing systems, and we've got a whole post about that. Two of the writing systems are pretty easy — each one has 46 characters (a consonant + vowel pair), and they're like the letters of our alphabet in that their only purpose is to represent a fixed sound. There are a handful of floating symbols (called diacritics) that can modify the pronunciation of several characters, and you can also combine certain characters to get new sounds, but all of that stuff will come pretty naturally.

The real hurdle of Japanese writing is learning the Kanji — 漢字 literally just means Chinese character. There are 2,136 daily-use kanji, a term that refers to charcaters which can appear in a newspaper without needing to have their pronunciations spelled out. There are several thousand more kanji you could learn, but you likely won't see them unless you're reading literature or consuming pretty technical content.

What really compounds the kanji problem, in my opinion, is the fact that each of the characters in Japanese has multiple readings — some "Chinese" and some "Japanese." This is  further complicated by the fact that the Chinese readings were borrowed from multiple Chinese languages over time. As a result, you end up with some real monsters — the character 生 (life) has over ten different ways it could be pronounced. You'll know when a Japanese or Chinese reading is called for, but even if you know you need a Chinese reading, you won't know which Chinese reading.

Mandarin writing system

Mandarin's writing is indeed daunting: from day one, you'll be looking at a wall of complicated looking characters.

What I think most window-shopping language learners overlook is how frontloaded the difficulty of Chinese characters are. They're a massive hurdle, and I don't mean to downplay that. You'll strugle to get over them... but, eventually, you will. Once you do, the payoff is massive. Mandarin words are just combinations of the same few thousand characters — once you've got a grasp on the characters, you'll be able to loosely understand what a word means even if you've never seen it before. We can't do that in English. I'm a university-educated native English speaker who works as an editor, but when I encounter new words, I've got no choice but to resort to the dictionary. Consider these words from the novel Jane Eyre: pinafore, moiety, soporific, poltroon.

Now, if you were reading the novel in Mandarin? Well, the translation of soporific drug is 催眠藥 — literally urge - sleep - medicine. Much easier!

Continuing down the avenue of struggle now, relax later: the vast majority of Chinese characters only get pronounced in one way. This consistency means convenience for learners. Some character components are phonetic in nature and others are semantic. For example, if we look at a word for mother 媽媽 (ma1ma5), what we see is that it has a meaning related to 女 (woman) but a sound like 馬 (horse, ma3). There are many such instances in Mandarin (how this works / a list of the phonetic "sets"), but it's something that you'll intuitively pick up over time. As a result, reading at an upper-intermediate/advanced level becomes quite smooth: whenever you encounter a new word, you'll likely have a rough idea of both what it means and how it sounds.

Before being able to enjoy that advantage, however, learners will be faced with a more immediate problem: should you learn the simplified characters or the traditional ones? The answer (at least for the early stages) depends on whether you're more interested in China (simplified) or Taiwan (traditional). After you're familiar with one set of characters, learning to recognize characters from the other set isn't especially difficult. There are a few characters that will leave you scratching your head the first time you see them (the simplified character for buy is 买, the traditional one is 買), but by and large, learning one set of characters means that you'll also be able to recognize the other set of characters.

So, with both Japanese and Mandarin, you'll struggle — but know that your efforts now will pay dividends for the rest of your life.

Korean writing system

The Hangul are nothing less than a work of art. Thousands of Chinese characters were replaced with just 24 letters: 14 consonants and 10 vowels which get mixed and matched to represent all of the sounds of Korean. You could knock that out in an evening, if you really wanted to.

The system does have its hiccups — there are quite a few spelling rules that you'll simply have to know in order to pronounce some words correctly. Many of Korean's inflections (for example, the present tense polite verb ending eoyo) will also shift slightly depending on the sounds that precede them, often in order to make a more natural/harmonious/easy-to-pronounce sound transition. You'll love the Hangul for an evening, then hate them for a month, then you won't think much about them anymore.

Then, the logic of the Chinese characters still resides in Korean's vocabulary, it's just not explicitly shown anymore. You'll find a lot of words that share sounds: the word for psychology is simlihag, and the word for school is haggyo. If you were to spell those words out in Chinese characters (hanja in Korean), you'd get 心理學 and 學校. As you can see, these seemingly unrelated words actually have something in common: 學, hag.

As such, there's a tradeoff here. You'll be able to learn the Hangul very quickly, but in exchange, you lose the ability to "x-ray" words as you can in Japanese and Mandarin.

Honorifics and communication

I don't know enough about Korean culture to do a fine comparison, so these sections will be very general in nature. Generally speaking, I think that learners will likely have an easier time adapting to the speech patterns of Mandarin-speaking countries than those of Korea or Japan.

Japanese and Korean vs Mandarin honorifics

Both Japanese and Korean have require the speaker to systematically alter their speech when a need to be polite or formal arises. We have an article on how that works in Japanese, but generally speaking, you'll be expected to use physically different language (different words, different inflections, different grammar) when speaking to someone of similar status to you versus someone higher in status than you. To effectively communicate in either Japan or Korea, you must constantly be aware of your relationship with the person you're talking to and how you both fit into  certain social hierarchies.

The usage of honorifics differs between Korea and Japan. To give a small example, Korean has honorific versions of (some) particles, but Japanese does not make this distinction. Then, to give a larger one: honorifics in Korea are predominantly hierarchical — you won't use honorifics with someone of the same age/status, even if you are distant from them; you will use honorifics with people who are older/higher in status than you, even if you're close frends. This isn't the case in Japan, where part of getting to know somebody is transitioning from more fromal speech to more casual speech.

In any case, honorifics will be a major hurdle for people studying Korean or Japanese, but are not really an issue for those studying Mandarin.

It's not that honorifics don't exist in Mandarin — you'll likely learn early on that there are two words for you, one formal (您) and the other informal (你). Having said that, Mandarin's approach to politeness feels a lot more like that of English: there are certain canned words and phrases that are seen to be more respectful than others, and you sprinkle them in to taste. It's more like adding spice to a dish, rather than choosing to dance a rumba instead of a waltz.

Japanese vs Mandarin communication

I've never lived in Korea and have not had many Korean conversations nor read any Korean books, or even watched a K-drama, so I'll leave it out of the below discussion.

One of the hardest parts about Japanese, for me, is that parenthetical information is often omitted.  Furthermore, communication is quite indirect. In the book I had that same dream again there's a line where the main character (a young girl) says something along the lines of the least beautiful way to express your thoughts is exactly as they come to you. Naturally, this isn't a six year old's observsation about the world: it's something that has been impressed upon her.

Similarly, there's a story (take it with a grain of salt) in which Natume Soseki (perhaps the most modern Japanese author) takes issue with how a student translated I love you into Japanese. More specifically, the student translated it directly; word for word. Soseki commented that doing so wasn't tasteful because being so direct was tantamount to a rejection of Japanese sensibilities. His suggested translation, given the story's context, was the moon is beautiful tonight, isn't it?

Now, Japanese people don't actually run around talking about the moon or speaking in riddles, but  the sentiment is there. Oftentimes I find myself struggling with Japanese not because I lack the words or grammar to express an idea, but because I'm certain a Japanese person wouldn't express it in the way that I'm about to. This is also an issue with books — parenthetical information often gets omitted from dialogue, so you have to read much more "actively" and connect dots for yourself. Sometimes I still get confused because I'm not sure who uttered a line of dialogue or what a particular comment is referring to.

Simply put, learning to communicate in Japanese is more than memorizing a bunch of words and grammar points. It's not mysterious or incomprehensible, but it is a different speed, and it will take you some getting used to.

Anecdotally speaking, I didn't encounter similar dificulties with Mandarin. I've found it much easier to make friends here in Taiwan than I did while I was in Japan. Similarly, whereas I've really struggled to find Japanese penpals on apps like Tandem or HelloTalk, I have the opposite issue with Mandarin — I can have as many conversations as I want to. (Maybe it's just me, and I happen to fit better into Taiwanese culture than Japanese culture.)

So which one is the hardest?

To rephrase my initial assertion in the to those… section, the hardest language is the one you’re least interested in learning. Pulling teeth for 100 hours will feel longer than 10,000 hours of a good time.

But, in my personal opinion:

• The difficulty of Mandarin is very front-loaded — most learners will be scared off early, due to the tones and characters, but if you find a strategy to get through those things, it’s largely downhill from there
• The difficulty of Japanese is more back-loaded — the abundance of high-quality resources and (seemingly) easier pronunciation make it easier to find your footing in the language, but the cultural preference for indirect communication means you’ll often find yourself in situations where you know every single word and grammar point in a sentence but don’t quite understand it… and that feeling will go on for a long, long time
• Korean (to my beginner eyes) seems to be somewhere in the middle — not needing to learn Chinese characters makes the language much more accessible, but the more complex grammar and much more complex spelling rules/pronunciation means it’ll take everything else in the language a bit longer to click

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