English — mustn't vs don't have to

These forms look similar but mean opposite things. You mustn't smoke here is a prohibition (it is not allowed), while You don't have to smoke here means there is no obligation (it is your choice).

How it works

✓ Rule

Use mustn't to say something is forbidden: You mustn't touch the exhibits. Use don't have to, don't need to or needn't to say something is not necessary: You don't have to come early — the door opens at nine anyway.

Examples

Prohibition vs. no obligation
Sentence	Meaning
You mustn't be late.	Prohibition — being late is not allowed.
You don't have to be early.	No obligation — arriving early is not required.
You mustn't tell anyone.	Prohibition — keep it secret.
You don't need to tell anyone.	No obligation — telling people is optional.
We mustn't waste water.	Prohibition — wasting water is not allowed.
We needn't hurry.	No obligation — there is no need to rush.

Good to know

⚠ Note

Don't treat mustn't as the negative of must in the sense of obligation. The opposite of You must go (obligation) is You don't have to go (no obligation), not You mustn't go, which forbids going. Also note that needn't is followed by the bare infinitive: You needn't worry, not You needn't to worry.