1. Must and Have To
Must and have to are both used for obligations—things you cannot choose not to do.
Must is used:
- When you have a strong feeling about the obligation.
- In formal or written English, especially on signs and notices.
- You must pay him back when you see him. –> I feel strongly about this.
- Passengers must have a valid ticket before boarding the train. –> Formal English, probably on a sign.
Have to is used when the obligation depends on rules or laws. For example:
- I have to wear a tie at work. –> It’s a rule; I don’t necessarily care about it.
- You have to be 18 before you can start driving. –> It’s the law, not what I think.
Want an easy answer? Use have to—you can never be wrong using have to for this meaning, but must can sound strange or even wrong, especially in spoken English. Have to is much more common in spoken English.
2. Mustn’t and Don’t Have To
Must and have to are not the same, but they are similar. The negatives mustn’t and don’t have to are completely different.
Mustn’t means that something is forbidden—you cannot do it. For example:
- You mustn’t smoke on the plane. –> It’s forbidden; you can’t do it.
- Employees must not use the Internet for personal reasons. –> It’s forbidden.
Don’t have to means that something is not necessary. If you don’t have to do something, you can choose to do it or not. For example:
- You don’t have to be here before ten. –> You can come before ten if you want, but it isn’t necessary.
- I don’t have to finish it today. –> I can leave it until tomorrow if I want to.
3. Must and Should
As we said before, must can be used to talk about obligations—things that you need to do.
Should is usually used to give advice or to express your opinion. For example:
- You shouldn’t say that to her. –> This is my advice—I think it’s a bad idea to say that to her.
- He should do more work if he wants to get promoted. –> This is my opinion, I don’t necessarily care if he does it or not.
But, must can also be used to give advice. If you use must to give advice, it has a very strong meaning. It is much stronger than should in this situation. For example:
- It’s a great film. You must see it if you can. –> Strong advice—I think it’s a really good film!
- You must go to the Louvre if you’re in Paris. –> I highly recommend it.
Shouldn’t can be used to give advice as well. When you use must to give strong advice, you can only use the positive form—you can’t use mustn’t to give strong advice.
4. Must, Have To and Should
Let’s review what you’ve studied.
You can use must for:
- Obligations which you feel strongly about
- Obligations in formal, written English
- Strong advice (must only)
- Saying something is forbidden (mustn’t only)
You can use have to for:
- Obligations which depend on rules or laws
- Obligations in spoken English
You can use should for:
- Giving advice
- Expressing your opinion