We use must when we think it is important to do an action:

• You must (= 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.

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:

• Police officers often have to work on Sundays. (= 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 on Sunday? ∼ 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:

• In Britain you must drive on the left. (or … you have to drive …) (= It is obligatory to drive on the left.)

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 inside public buildings. (= Smoking is against the law in public buildings.)
• In football you mustn’t touch the ball with your hands. (= It is against the rules of football to touch the ball with your hands, unless you are the goalkeeper.)

We use don’t have to to say that people are not obliged to do something:

• I don’t have to go to work tomorrow. (= There is no obligation for me to go to work tomorrow.)
• You don’t have to pick me up at the station. I can take a taxi. (= There is no obligation for you to pick me up at the station.)

In questions we usually use do/does … have to (not must) to ask if something is obligatory or important:

• Does he have to go to work tomorrow? ∼ No, he doesn’t.
• Do I have to wear a suit to the party?