must的否定是Mustn't and don't have to，很多同学常常混淆不清

Must, mustn't, have to 和 don't have to 都是情态动词，后面跟动词原形（其实是省略to的 动词不定式 to do形式）.

和多数情态动词不同， have to and don't have to 要与主语保持主谓一致，而且有过去时态. 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 都指义务，也就是说，它们都用来解释需要做什么.

You must stop smoking. It will kill you.

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

must指义务, 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一般解释某事我们没有义务做，完全由我们自由选择，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!

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