"Don't have to" and "Mustn't"


"Have to" and "must" usually mean the same thing: there is no choice about something, and you are forced to do it. However, the negatives of these two modals mean different things. "Don't have to" is different from "mustn't":
Don't have to
means that something is not necessary, but you can do it if you want to -- it's up to you. "You don't have to pass the test."
means

"It's OK if you pass the test, but it's also OK if you don't pass it."
Mustn't
means that something is not allowed; it is forbidden, and you definitely shouldn't do it. "You mustn't smoke in the classroom."
means

"Smoking is not allowed in the classroom."