I have to tell him not to hurt himself.中的not可以用don't吗?为什么?

问题描述:

I have to tell him not to hurt himself.中的not可以用don't吗?为什么?
那若是这个句型呢:I was driving quickly.He said to me"You'd better not do it."

不可以
因为这是固定搭配
tell sb to do sth 告诉某人做某事
tell sb not to do sth 告诉某人不要做某事