Question

什么时候用be动词，什么时候用助动词，什么时候都不要用

Answer 1

"be doing" 通常是正在进行的标志，可以是过去、现在、甚至将来。
比方说
When my friend came yesterday, I was doing my homework. （过去）
I'm doing my homework now! （现在）
i'm afraid that I will be doing my homework tomorrow when you come. (将来)

助动词通常用在一般态疑问句或否定句中
比如：
do you like him? No, I don't (like him).
Did you go to China last year? No, I didn't (go to China last year).

助动词也可表强调：
I do like her. （我的确很喜欢她）

在一般态的句子中，两者都不用。
比如：
I like him. She likes him.