1. We use must when the speaker thinks it is necessary or important to do an action:
- You must go. (= It is important that you go.)
We make negatives, questions and short answers like this:
- You mustn’t go.
- Must you go? ~ Yes, I must.
2. We use have to to talk about an action that is necessary because of rules or laws, or because someone obliges us to do it:
- Doctors sometimes have to work on Sunday.
(It is in the rules of their work.)
We make negatives, questions and short answers with a form of do:
- Teachers don’t have to work on Sunday.
- Do you have to work today? ~ No, I don’t.
In positive sentences we can often use must and have to with little difference in meaning, because many things are important both because we think so and because there are rules:
- You must work hard in order to succeed (or … you have to work… ).
Note the difference in meaning between mustn’t and don’t have to. In negative sentences we often use mustn’t to say that something is against the rules, or against the law:
- You mustn’t smoke on buses. (Smoking is against the rules.)
- In football you mustn’t touch the ball with your hands. (Touching the ball is against the rules.)
We use don’t have to to say that people are not obliged to do something:
- In Britain, people don’t have to carry a passport with them. (= People are not obliged to carry one.)
- Nowadays pupils do not have to learn Latin at school. (= They are not obliged to learn it.)
In questions we usually use do/does … have to (not must) to ask if something is obligatory or important:
- Does Michael have to get up early tomorrow?
- Do we have to wait here?