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English definition of “must”

must

modal verb  /mʌst, məst/ (must) us  

must modal verb (BE NECESSARY)

used to show that it is necessary or important that something happen in the present or future: Seeing what others have and she lacks, she believes that she must have more. We must not surprise them (= it is wrong, dangerous, or forbidden). If you say that you must do something, you can mean that you have a firm intention to do something in the future: I must call my sister later. Must is sometimes used for emphasis: I must admit I enjoy these movies.

must modal verb (PROBABLY)

used to show that something is likely, probable, or certain to be true: Death must be better than this. "It must have been fun." "No, it wasn’t."
(Definition of must modal verb from the Cambridge Academic Content Dictionary © Cambridge University Press)
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