Must express obligation and a strong necessity, it is followed by a verb without “to”.
Have to is an equivalent of Must but not as strong.
- Muslims must pray (perform salah) five times a day.
- Students have to do their homework.
Must not or Mustn’t expresses prohibition, something forbidden to do.
- You must not sit on grass.
- You mustn’t smoke in a cinema.
Needn’t or don’t/doesn’t have to express the lack of necessity, something which is not necessary.
- You needn’t buy bread. We have plenty.
- You don’t have to buy bread.
Must also expresses a logical conclusion.
- Rachel is absent from school today. She must be ill.
- You must be kidding me.