Quoting Man. Bear. Pig.:" They say English is the hardest to learn. Then I look at languages like Chinese or French, and I think.... ... [snip!] ... I think.... how is that possible. Yeah, if they say it's the hardest, then I commend those who are able to learn it, for sure."
Inflectional languages are hard for us to comprehend, like Chinese. Ma with a rising tone means mother, and Ma with a falling tone is something bad, I think prostitute? Lol I cant remember.
That being said, while inflections are hard for us to comprehend the structure and the way that english is a "borrowed" language is where problems start. We borrow from so many other languages that English really isn't English in it's true form, if that makes sense. It's a melting pot of languages with structures that change all of the time and it's confusing to non native speakers.
For example, other langauges have double negatives. In English we do not. But non native speakers can be confused when saying certain sentences or understanding our negation when it doesnt follow the form of their own language and have double negatives. Tense markers... some languages dont have the -ed ending or other tenses. To say "walked" in one language (cant recall) you use a negation instead of the -ed ending. Many languages dont have tense markers, it's just a negation or even double negation.
Most sentences of ours follow the SVO form. Which is subject, verb, object. Example "The boy (subject) threw (verb) the ball (object)." Or "Will you (subject) teach (verb) me how to paint (object)?" We have compound sentences which can confuse speakers. Such as "Will you teach me how to paint, additionally, will you please be patient if I suck greatly?" It's hard for non native speakers to understand compound sentences.
Then we use tricks. Such as the SVO "The boy threw the ball" changed into OVS "The ball was thrown by the boy." It's a mental tactic... when you use the OVS form its usually to take blame off of the subject. It takes the focus off of them. Read a newspaper, they do it a lot to take blame off of a subject. Instead of "Kesha was allegedly found smoking marijuana," the paper would probably say "Marijuana was allegedly consumed by Kesha." It is a mind game to make people take blame off of the subject. Its really hard for non native speakers to understand the other form though.
Or other forms, like "Running fast, the boy broke the race record." Which is VOS. Or "Running fast, the race record was broken by the boy." Which is VOS. Again, it's a head game. Depending on how "elite" you want your writing to be, you write a certain way to your audience. Its hard for non native speakers to understand!
The list goes on.... there are MANY things that come natural to us but its really hard for other speakers because their language is so different in structure. Like we don't have question markers at the end of our sentences, Spanish can.
I loved studying Linguistics... it bugs me because of how complicated it can get but at the same time... its SO fascinating!!!