1. If it were to rain tomorrow, what will you do? -- Change order of these two. "What will you do if it were to rain tomorrow?" Notice that there isn't a comma in the corrected sentence.
2. I would give it to medicine research, if I got a lot of money. -- Change order of these two. It should be "If I got a lot of money, I would give it to medicine research."
3. "This old amn had 2 million dollar hat he gave to charities" sound more correct.
4. This is okay, I guess.
5. "He doesn't know if he should bring a present when invited..." This sentence is a little incomplete though, but that's just my opinion. It's not wrong saying this as far as I'm concerned.
6. This is okay.
7. I don't understand what you want to say in this sentence. It doesn't make sense. "If you don't know anyone, you could talk to Tom"? Maybe? I don't know.
8. Okay.
9. "......, I would take a long walk before I go to bed". Add the subject there. I just put in "I" because I assumed that was what you meant.
10. "I can't sleep AT night before taking a big exam."
11. "I don't have a gift. What if everyone else brings a gift?"
12. and 13. Okay. In 12 you don't necessarily need the "the" before "public", but it can be written with "the" too.
14. "I really need it because I have a math test". You don't need the "of" in "because of".
15. Okay.
16. Don't need the comma there.
17. Change "are" to "have". Are is present tense, but since you use "happened" which is past tense of "to happen", you have to use "have" in front of "happened".
18. Okay.
19. Change "aren't" to "doesn't" and take away the "in".
20. Add an "s" at the end of "sometime". Should be "sometimes".
(^o^)v