must modal verb, noun - definition in the British English Dictionary & Thesaurus - Cambridge Dictionaries Online (US)

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must

modal verb uk   us   strong /mʌst/ weak /məst/ /məs/

must modal verb (NECESSARY)

A2 used to show that it is necessary or very important that something happens in the present or future: Meat must be cooked thoroughly. I must get some sleep. You mustn't show this letter to anyone else. Luggage must not be left unattended (= it is against the rules).formal Must you leave so soon?formal "Must I sign this?" "Yes, you must." If you say that you must do something, you mean that you strongly intend to do something in the future: I must phone my sister. We must get someone to fix that wheel. I mustn't bite my nails. used for emphasis: I must say, you're looking extremely well. I must admit, I wasn't looking forward to it.B1 If you tell someone else that they must do something pleasant, you are emphasizing that you think it is a good idea to do that: You must come and stay with us for the weekend. We must meet for lunch soon.
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must modal verb (PROBABLY)

B2 used to show that something is very likely, probable, or certain to be true: Harry's been driving all day - he must be tired. There's no food left - we must have eaten it all. When you got lost in the forest you must have been very frightened. "You must know Frank." "No, I don't."
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must

noun [C] uk   us   strong /mʌst/ weak /məst/ /məs/ informal
something that is necessary: If you live in the country a car is a must.
(Definition of must modal verb, noun from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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