From Longman Dictionary of Contemporary Englishmustmust1 /məst; strong mʌst/ ●●●S1W1 modal verb (negative short form mustn’t)1 (past tense had to)MUST to have to do something because it is necessary or important, or because of a law or order → have to, obligeAll passengers must wear seat belts.It’s getting late. I really must go.You must work hard.We must all be patient.Must I pay now?For the engine to work, the green lever must be in the ‘up’ position.Accidents must be reported to the safety officer.RegisterIn everyday English, people usually say someone has to or has got to do something rather than say they must do something, which can sound slightly formal or emphatic:Everyone has to wear a seat belt.I‘ve got to go at four o’clock.Do I have to pay now?2used in negativesentences to say that something should not happen, because of a rule or law or because of the situationYou mustn’t talk to your mother like that.This book must not be removed from the library.We must never forget how much we owe to these brave men.No one must disturb him while he’s sleeping.3MUSTused to say you think something is very likely to be true or very likely to have happenedSam must be nearly 90 years old now.His new car must have cost around £20,000.You must have been really upset.There must be something wrong with the engine.Karl must’ve seen ‘Star Wars’ six or seven times.4SUGGEST spoken used to suggest that someone should do something, especially because you think they will enjoy it or you think it is a good ideaYou must come and stay with us in London sometime.‘We must do this again, ’ he said. ‘I’ve enjoyed it thoroughly.’5especially British English spoken used to say that you intend or want to do somethingI must call her tonight.
6 →I must admit/say/confess7 →(why) must you ...?8 →a must-have/must-see/must-read etc9 →if you must (do something)10 →if you must know → you must be jokingat joke2(2)GRAMMAR: Comparisonmust• You say that you must do something: I must get a new key.• Don’t use ‘to’ after must. ✗Don’t say: I must to get a new key.• You use must not when saying someone should not do something, for example because it is dangerous or wrong: You must not leave without telling me where you’re going.• You say that something must have happened, or that someone must have done something, when you are fairly sure that this is true, based on what you know: They must have caught an earlier train.have to• You say that you have to do something: She has to do some shopping.• You use don’t have to when it is not necessary for someone to do something: You don’t have to leave yet.• You use had to instead of ‘must’ when talking about the past: He had to get up early the next day.THESAURUSmust do something used when saying that it is very important that someone does something, because someone in authority or a rule says this, or because of the situationYou must be home by midnight.He must not smoke in here.I must remember to call her.have to do something to need to do something because it is necessary or importantI have to go home early.She has to talk to him first.be obliged to do something to have to do something, because of a rule or law, or because of the situation you are inMembers of parliament are obliged to declare all their financial interests.He was obliged to admit defeat.be required to do something formal to have to do something because of a rule or law – used especially in written notices and officialdocumentsNew students are required to register with a doctor.have an obligation to do something (also be under an obligation to do something) to have to do something because it is the duty of someone in your position to do it, or because you have officially agreed to do itThe landlord is under an obligation to keep the building in good repair.be forced/compelled to do something to have to do something that you do not want to do, because you are in a situation that makes it impossible to avoidShe was forced to retire early due to ill health.