Mustn't vs Don't Have To - Prohibition - Formation, Use, Examples & Video

Q: What is the real difference?

They are opposites in feeling. Mustn't = it is prohibited (you have no choice but to not do it). Don't have to = there is no obligation (you are free to choose).

Q: How do I remember which is which?

Think: mustn't = a red light (forbidden); don't have to = a green light that you can ignore (not necessary).

I Swear English › Modal Verbs › Mustn't vs Don't Have To - Prohibition

How to form the mustn't vs don't have to

Mustn't is a prohibition. Don't Have to is optional.

Mustn't talks about a prohibition in the present or future. A rule. A norm. A strong obligation. Something that is not permitted.
For example: You must not smoke in a petrol station. It is prohibited by law.
You mustn't be rude to your parents, you must show more respect.
People mustn't eat too much sugar and salt if they wish to live a long life.

Don't have to talks about things that are not necessary, but that you can do if you want. The action is possible and optional, but not obligatory.
For example: You don't have to donate money to charity, but it is a good thing to do.
You don't have to come to the party, but I would really appreciate it if you did come.
You don't have to do it my way, but if you do it your way, you are going to have trouble.

The following video explains the difference between Mustn't and Don't Have to:

When to use it

Mustn't — prohibition. It is not allowed; don't do it. You mustn't smoke here.
Don't have to — no obligation. It isn't necessary, but you may if you wish. You don't have to come — it's optional.

Signal words & time expressions: it's forbidden, it's not allowed, it's optional, it's up to you

Watch the video lesson

Video transcript

Hi there. Mustn't and don't have to. Notice in the positive, must and have to are very similar. In the negative they are completely different. You must not, a prohibition. You don't have to. It's not necessary. It's optional. It's voluntary. OK. So, you mustn't eat poison, or you will die. You don't have to eat chocolate, but you can eat chocolate if you want. So, you don't have to, you can if you want. Erm.. people work Monday to Friday and sometimes they work on Saturday. Sometimes it's voluntary and sometimes it's not. OK. So, if it's voluntary you don't have to work on Saturday, but you can if you want. It's that simple. You mustn't, it's prohibited. You don't have to, you can if you want, it's possible but it's not necessary. In another video I... we will look at mustn't and don't have to in the past, because notice mustn't is only for present and future. So thanks for watching and I'll see you in another video. Bye for now.

Common mistakes to avoid

Not quite…	Better
It's optional, so you ~~mustn't~~ come.	It's optional, so you don't have to come. — mustn't would mean it's forbidden!
Smoking is banned, so you ~~don't have to~~ smoke.	… you mustn't smoke. — use mustn't for something prohibited.

Frequently asked questions

What is the real difference?

They are opposites in feeling. Mustn't = it is prohibited (you have no choice but to not do it). Don't have to = there is no obligation (you are free to choose).

How do I remember which is which?

Think: mustn't = a red light (forbidden); don't have to = a green light that you can ignore (not necessary).

Continue the course

This lesson is part of our free English Modal Verbs course — work through it in order or jump to what you need next.

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