Must expresses 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 (a contraction) 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.
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