Mustn't means it's not allowed: You mustn't smoke here. Don't have to means it's not necessary, but you can if you want: You don't have to come. They look similar but mean very different things.
Same situation, two choices. Pick an example.
You mustn't talk during the exam.
Forbidden: don't do it.
You don't have to answer every question.
Optional: you can do it, but you don't need to.
| Rule | Mustn't | Don't have to |
|---|---|---|
| Meaning | Not allowed | Not necessary |
| Your choice? | No: don't do it | Yes: do it if you want |
| Similar to | can't, not allowed to | don't need to, needn't |
| Example | You mustn't park here. | You don't have to park here; there's space at the back too. |
You _____ touch that wire. It's dangerous!
Paste an email or an essay. Clarigram marks the mistakes you actually make, shows the rule behind each one as a picture, and builds your practice from them.
Yes: You needn't come means You don't have to come. Needn't is more British and a little more formal.