Listen more, speak more, and it will be self-corrected.

The first focuses on the action itself which is done, and the second on the fact that it is finished.

"如果" is more like "if" and "假如" is more like "provided", I guess. But generally, they're just the same. Another one - "要是" is more oral and with stronger personal emotions.

I sleep.

No differences. It's just the problem of rythm. And it's up to one's preference. Chinese just doesn't have a strict structure.

Some Chinese thought some Chinese caracters are too complicated to write, and simplified them. People in China now use simplified Chinese as daily language, except in Hong kong and Taiwan. However, if you want to read the old books written by ancient Chinese writers, you'd better read the traditional Chinese caracters, otherwise, you may be misunderstand.

People often say something(four words) like a 成语, but actually it never can be a 成语. Because the real 成语 are created in or modified from the works of ancient great writers.

I don't think a foreigner need to learn so many chinese languages, Mandarin plus Cantonese is quite OK.
I know Changsha hua too..