How to ask questions in Chinese. 吗, 吧

1. Use "吗" 
If it is a simple statement, usually put "吗" at the end of the sentence, it would become a question directly.

e.g.
他明天来                       ->  他明天来吗？
He will come tomorrow   ->  Will he come tomorrow?

她喜欢看电影                       ->  她喜欢看电影吗？
She likes watching movies.       Does she like watching movies?


2. Use "吧"
You can use "吧" instead of "吗" in the questions similar to those above. But it indicates that you expect it is right.

他明天来吧?   她喜欢看电影吧？


3. Not to use "吗"
a.When you ask about what/when/how/where/how many/how much etc., don't use 吗. For instance:
他什么时候来?  她喜欢什么?

b. When it is an optional question, don't use 吗. For instance:
他明天来不来?  他明天来还是后天来?  她喜不喜欢看电影?