When you get past basic grammar, things do get quite tricky. There are tons of grammatical forms that use "koto" and "mono," for example, and keeping them all straight and distinct from each other takes a fair bit of effort.

Two things that are especially difficult for an English speaker are wa/ga and how the verbs for giving/receiving are so complicated, and made even more so by needing to use the right politeness level.

Just learning Chinese seriously, and I don't think Chinese is a particularly difficult language, aside from the two aspects @lproven mentioned (tones and writing system).

But I would agree with one of my teachers, that Chinese is "longer" not "more difficult". Or, as another put it: "Most Chinese words are only one or two letters long. But you need to learn 3000 letters."

In most other aspects, Chinese is surprisingly simple and really logical:

- around 20 to 30 main grammar constructions (sentence forms like conditionals, etc.)
- no conjugations or declinations
- new, unknown words can often (not always) be worked out by the meaning of the characters
- Chinese only has around 400ish syllables, which can come in 4 tones (plus the neutral tone), which makes it less than 2000 "sounds" one needs to be able to produce (compared to >15000 or so in English).

The latter is also a difficulty because it means listening comprehension is extremely difficult, and while I can read (only around 400 characters so far, so a long way to go still) and speak, my listening comprehension is not yet up to the same level (one of the "long" language aspects).

Danish is sufficiently hard to learn that even children of native speakers take longer to learn it properly than for any other language. The most likely cause is the bewildering array of almost identical vowel sounds.

Early vocabulary development in Danish and other languages: A CDI-based comparison | Journal of Child Language | Cambridge Core

Early vocabulary development in Danish and other languages: A CDI-based comparison - Volume 35 Issue 3

Cambridge Core (www.cambridge.org)

The language itself, its grammar and so on, is actually one of the simplest I've ever even learned a few sentences of.

The thing is that it's an N-dimensional problem. The things that make 1 particular language hard are often unique to that language (or family thereof), and in other ways it may be extremely simple.

At a basic level Japanese grammar is also very very simple. But counting is very complicated (there are no plurals, for a start, AIUI) & register (politeness, manners) are both very complex & very very important.

(AIUI, the main thing that differentiates Malaysian & Indonesian is register.)

Register is a big deal. It doesn't matter much in English. We even got rid of thee & thou & the associated verb forms.

You need dozens of axes to define the bits that are difficult/easy.

Some of the famous ones are only difficult in a few ways.

The really tricky languages are tricky in _lots of different ways at once_.

If Chinese is really hard in 2 ways, say (tones & script), then some are hard in 10 or 15 ways at once.

There is a fascinating visualisation exercise in here somewhere.