Well opinions on that question vary. The US Department of State's Foreign Service Institute has given major foreign languages a rank in order of difficulty to learn for native English speakers. Although the ranking seems to have changed over time from a scale of 1-4 (4 being hardest) to a scale of 1-3, Chinese is invariably given the highest (hardest) score.
But, if you want my humble opinion (and what else is a blog?), I think the reading/writing Chinese hanzi characters is what makes the language hard, and speaking is relatively easy—-kind of. I'll break it down below, but I have to say this first: The best way to learn Chinese slowly is to try to tackle the hanzi characters from the beginning. I've got a whole little article on the subject of hanzi as a pedagological millstone around students' linguistic necks, but that's for a later post.
I've created my own scale of 1-10 where:
1 = as easy as I imagine a foreign language could be to learn
10 = as hard as I imagine a foreign language could be to learn
How Hard is Chinese?
Grammar: 3 (pretty easy)
Word order is often the same between Chinese and English
Pronouns. No distionction between he/she/it. And no distinction between cases (he, she, her, it, its). tā 他= he, him, she, her, it. To make the possessive form add the "de 的" particle (tā de 他的).
No inflected cases of any kind (like in German or Slavic languages)
No plural nouns. "Apple" and "apples" are both are translated píngguǒ 苹果. The only exception is the plural suffix "men" for pronouns (wǒ 我= I, wǒmen 我们= us; tā 他/她/它= he/she/it, tāmen 他们= them). But as you see, even that doesn't change depending on usage.
No verb tenses to speak of. Do, does, did, doing, done are all translated as zuò. Tense is shown by time markers ("yesterday I do") or tricky little particles ("I am going to go now" = wǒ zǒu le 我走了(I go + particle to show change in states or situation).
No articles. "A book," "the book," the Chinese don't care. Just say "book" (shū 书).
1.Measure words. Nothing could be perfect. Even though there's no plural or articles, they had to add those pesky little measure words that basically serve no linguistic purpose. There is no way to say "One book" in Chinese without saying the correct measure word (běn 本). So, really what you're saying in Chinese is "One bound-thing book" (yì běn shū 一本书). Or for table, "One flat-surfaced-thing table" (yì zhāng zhuōzi 一张桌子). My Chinese friends avidly condone measure words as a way of categorizing and organizing the world and telling you the shape of the thing you're going to be talking about. I reply with, "There's no reason for me to know the shape of the thing you're about to say because you're about to say the name of the thing itself which will be infinitely more useful for my imagining its shape." I then hear crickets chirping for a few seconds and we change subjects. My theory as to the practical use for measure words has to do with the myriad homonyms in Chinese, but that's another post.
2.Adjectival phrases. Not all the word order is the same. The biggest difference, in my experience, is saying things like, "The person reading a book" in Chinese needs to be said, "The reading book person" (kàn shū de rén 看书的人).
Adverbs of place.Similar to the adjectival phrases, "Behind the shop" needs to be said "At the shop behind place" (zài shāngdiàn hòumian 在商店后门).
3.Tricky little particles. It's a real trick to master when to throw in a "ba 吧" or a "ne 呢" or a "de 的" or a "le 了." But if I'm not sure I just pick my favorite and at least other foreigners think I know what I'm saying.
4.Word order very important. To pay the piper for all the perks of not having any cases or tenses, the Chinese depend very much on word order to convey meaning. Although it's not as important in conversational Chinese as the textbooks would have us believe, throwing two words in the wrong order can absolutely stump most of them. I'll try to think of a real-life example soon (it seems like it's happened so often, yet they all escape me).
Vocabulary: 5 (medium)
Pros (In addition to the above grammar pros):
1.Little chunks. Since the Chinese language is made of a lot of characters (each with their own meaning), their compound words are combinations of smaller words. For example, "motivation" (dòngjī 动机) when broken down into it's two characters means "move machine." Also, "resolved, determined" (juéxīn 决心) means "decide heart." Modern words also need to use known characters in new combination. The word for "blog/blogger" (bókè 博客) has the same first character as the first character of "doctor/Ph.D." and the second is "guest." So, because of this bite-sized approach, vocabulary learning can gain a kind of critical mass where you're not actually learning new pieces but, learning a new word means arranging old pieces in a new way. This can be made easier if you organize your known vocabulary using what I call a "hanzi web" (my next post).
1.So many homonyms. Since there are so few possible syllables, and only 4 tones (I never thought I'd say "only"), there are so many words that sound exactly the same. The only way to distinguish them in writing is that they each have a different hanzi character. In speaking, context determines their meaning. The winner I think is shì with at least 40 different characters for the same pronunciation. There's even a poem in ancient Chinese where someone wrote the whole poem using only words that were pronounced "shi" with different tones.
2.The tones. For every word you have to remember the right tone. This is just straight memorization. Its like learning two words for every one. The best way to remember the tones is to make a little mnemonic device for yourself (a later post).
Pronunciation: 6 (medium-hard)
1.Limited pain. There are only a few hundred possible syllables in Chinese (strangely, people can't agree on the exact number, but it's around 400). After you learn to pronounce those, you're done. You can say anything in Chinese.
1.The tones again. Even though there are only 4 tones, you still have to a) remember the right tone to say, b) say that tone correctly, c) adjust for any tone changes. The tones do change in relationship to each other. There are rules, but you still have to remember the rules.
2.The secret tone. Most people agree there is a 5th, neutral tone. I've found this to be mostly a lie. a) How am I supposed to say something with no tone at all? Really fast? Really quiet? b) that hanzi character does have a tone and all the Chinese know what it is. If they say something really slowly it comes right out. For example, you'll never hear a Chinese person say "you all" (nǐmen 你们) slowly without putting a 2nd tone on the "men." It's true that ma, ba, ne and other little particles really have no tone. But that doesn't mean you're allowed to say them any way you want. For example in "let's go" (zǒu ba 走吧) you have to say the ba like it's a first tone. There are rules, but it's still a hassle.
3.Some new sounds. As should be expected when learning a foreign language, there are some new sounds for speakers of English learning Chinese. For vowels the newest sound is the umlaut ü but the back "i" sound in pinyin "shi" and "si" can be a little tricky too. For consonants the biggest challenge is the "r" which is said a lot of diffrent ways (depending on whom you ask) including sounding just like our "r." There are also two ways to say our "sh" and "ch" in Chinese. For example the "sh" sound in "thank you" (xièxie 谢谢) and the "sh" sound in book ( shū 书) are different.
Listening: 7 (hard)
I can't think of any
1.So many homonyms. Even if I know the word, sometimes I can't understand it because my mind immediately chooses to understand one of the syllables wrong. For example, I kept hearing "math" (shùxué 数学) as "tree" (shù 树) study even though I knew that should never be anyone's major.
2.So many synonyms. There are so many words for the same thing in Chinese. But people say, "it's the same in every language." No. Well, yes. But Chinese is different (probably because it's been around a lot longer than…well, most any other currently spoken language). They've had thousands of years to accumulate vocabulary. The various dialects and regions only compound the problem. I'll give one example. In English I only know one way to say, "I speak English." I guess I could say, "I speak American English" but that's it. In Chinese I know of 10 ways to say, "I speak Chinese." 2 words for speak (shuō 说, jiǎng 讲) and 4 words for Chinese (zhōngwén 中文, zhōngguóhuà 中国话, hànyǔ 汉语, guóyǔ 国语, pǔtōnghuà 普通话). Although some of those mean Mandarin as opposed to just Chinese, people use them interchangeably. This means when I'm speaking I have all sorts of fun options to say. But for listening, if I didn't happen to learn all the different ways something could be said, I'm easily stumped.
3.So many dialects. While common wisdom states that "A billion people!" speak Mandarin Chinese, a recent article said that ain't the case. This helps explain why so many people in my first two years living in China didn't speak Mandarin to me. They didn't know it. It makes me feel a little better for not understanding them, but still. Toward the middle of my first year I actually started asking people I couldn't understand, "Are you speaking Mandarin" just to try to get the comprehension burden off myself.
4.So many accents. Even if people speak Mandarin, their accent may vary wildely from the standard. I still remember a time in a Kunming drugstore when a guy came up to say to me, "no one is watching your bike" and I kid you not (and I confirmed this with other people later) what he said sounded EXACTLY like "no one is watching your eggplant." He was speaking what he thought was Mandarin (and I give him full marks for that) but his accent was some amalgamation of his local dialect and Mandarin.
5.Reduced forms. Simillar to the English reduction of "I would have been killed" to "I wouldabeen killed," Chinese speakers often drop one or more syllables of common words. For example reducing "zuótiān wǎnshàng shuìjiào de shíhòu 昨天晚上睡觉的时候" to "zuó wǎn shuìjiào shí 昨晚睡觉时." If you don't know that not all syllables are created equal, and some can be dropped, understanding those reduced forms is basically impossible.
Reading: 8 (really hard)
Reading is easier than writing because it's receptive not productive. Conventional wisdom says to read a newspaper you need to know 3000 characters. That's not 3000 words but 3000 little pieces of words (see vocabulary pros). And the only way to learn those characters is rote memorization. Many are made of several smaller pieces (radicals) and that can give you a clue as to how it MIGHT be pronounced. But really, it feels a bit like memorizing the Periodic Table of the Elements, except longer.
Writing: 9 (really, really hard)
The only reason this isn't 10 is because I'm in mainland China and they use "simplified" characters. Taiwan and Hong Kong writing is a 10.