Must vs. have to Both Must and have to express obligation or necessity, but there are some small differences: - We use must when the speaker thinks it is necessary or important to do an action: Must you go? ~ Yes, I must. You must go. (= It is important that you go.) - 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.)