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don't have to

Don't have to = Do not have to

We use don't have to to say that there is no obligation or necessity to do something.

For example: "You don't have to do the exercises at the end of this page."

Mustn't = must not

We use mustn't to show that something is not allowed. When you use mustn't you are telling people not to do something. It has the same force as don't , as in: Don't do that!

For example: "You mustn't drink if you're going to drive."