Definition of must modal verb from the Oxford Advanced American Dictionary

      

    must

     modal verb
    modal verb
    NAmE//məst//
     
    , NAmE//mʌst//
     
     
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  1. 1 used to say that something is necessary or very important (sometimes involving a rule or a law) All visitors must report to reception. Cars must not park in front of the entrance (= it is not allowed). (formal) I must ask you not to do that again. You mustn't say things like that. I must go to the bank and get some money. I must admit (= I feel that I should admit) I was surprised it cost so little. Must you always question everything I say? (= it is annoying) “Do we have to finish this today?” “Yes, you must.” Note that the negative for the last example is“No, you don't have to.”
  2. 2used to say that something is likely or logical You must be hungry after all that walking. He must have known (= surely he knew) what she wanted. I'm sorry, she's not here. She must have left already (= that is the likely explanation).
  3. 3 used to recommend that someone do something because you think it is a good idea You simply must read this book. We must get together soon for lunch. Grammarmodal verbsThe modal verbs are can, could, may, might, must, ought to, shall, should, will, and would. Dare, need, have to, and used to also share some of the features of modal verbs.Modal verbs have only one form. They have no past or present participles and do not add -s to the 3rd person singular form:He can speak three languages. She will try and visit tomorrow.Modal verbs are followed by the infinitive of another verb without to. The exceptions are ought to and used to:You must find a job. You ought to stop smoking. I used to smoke, but I quit two years ago.Questions are formed without do/does in the present or did in the past:Can I invite Mary? Should I have invited Mary?Negative sentences are formed with not or the short form -n’t and do not use do/does or did:You shouldn't invite Mary. The error will not have affected our results.You will find more help with how to use modal verbs at the dictionary entries for each verb.
  4. Grammarmust / have (got) to / must not / don’t have tonecessity and obligationMust and have (got) to are used in the present to say that something is necessary or should be done.Have to is more common, especially in speech:You have to be home by 11 o’clock. I have to wash the car tomorrow. I have to collect the children from school at 3 o’clock.Must is stronger and more formal:All nurses must wear uniform. Changes must be recorded in the log book.There are no past or future forms of must. To talk about the past, you use had to and has had to:I had to wait half an hour for a bus. Will have to is used to talk about the future, or have to if an arrangement has already been made:We’ll have to borrow the money we need. I have to go to the dentist tomorrow.Questions with have to are formed using do:Do the children have to wear uniforms?In negative sentences, both must not and don’t have to are used, but with different meanings. Must not is used to tell someone not to do something:Employees must not smoke in the building.The short form mustn’t is rare and very formal:You mustn’t leave the gate open.Don’t have to is used when it is not necessary to do something:You don’t have to pay for the tickets in advance. She doesn’t have to work on weekends.certaintyBoth must and have (got) to are used to say that you are certain about something. Have to is the usual verb in this meaning:He has (got) to be the worst actor on TV!If you are talking about the past, use must have:Your trip must have been fun!Idioms
    if you must (do something)
     
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    used to say that someone may do something, but you do not really want them to “Can I smoke?” “If you must.” It's from my boyfriend,if you must know.
    must-see/must-read/must-have, etc.
     
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    used to tell people that something is so good or interesting that they should see, read, get it, etc. Sydney is one of the world's must-see cities. The magazine is a must-read in the show business world. This is on my must-do list. a must-have for any fan
See the Oxford Advanced Learner's Dictionary entry: must