Must, mustn't, have to 和 don't have to 都是情態動詞，后面跟動詞原形（其實是省略to的 動詞不定式 to do形式）.

和多數情態動詞不同， have to and don't have to 要與主語保持主謂一致，而且有過去時態. 如:

He must finish his work before going home.

Everyone mustn't smoke here.

You have to listen to your mother.

A child had to help in the farm.

He doesn't have to go.

must和have to 都指義務，也就是說，它們都用來解釋需要做什么.

You must stop smoking. It will kill you.

Every child has to go to school. It's the law.

must指義務, 但mustn't一般指“禁止”做某事，而不是“不必”如：

You mustn't talk to your teacher like that.

She mustn't leave the house until I come home.

We mustn't drink before driving.

Don't have to一般解釋某事我們沒有義務做，完全由我們自由選擇，如：

You don't have to go to school if you are sick.

She doesn't have to come to the party, if she doesn't want to.

I don't have to know all the moves to win the game!

總而言之，must是必須，don't have to是不必！大家都清楚了吧？