Definition of must modal verb from the Oxford Advanced Learner's Dictionary

      

    must

     modal verb
    modal verb
    BrE BrE//məst//
     
    ; NAmE NAmE//məst//
     
    ; BrE strong form BrE//mʌst//
     
    ; NAmE strong form NAmE//mʌst//
     
    Verb Formsmust notmustn't
    BrE BrE//ˈmʌsnt//
     
    ; NAmE NAmE//ˈmʌsnt//
     
     
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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. (especially British English) 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.’ Express YourselfAsking about obligationWhen you are unsure about what is expected of you in a situation, you can ask about obligations: What time do we have to be home? Are we supposed to show our ID cards? Is it necessary to apply for a visa? Is there a legal obligation to wear a bike helmet here?
  2. 2  used 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 must be the explanation). Grammar Pointmust / have (got) to / must not / don’t have toNecessity and Obligation Must and have (got) to are used in the present to say that something is necessary or should be done. Have to is more common in North American English, especially in speech:You must be home by 11 o’clock. I must wash the car tomorrow. I have to collect the children from school at 3 o’clock. Nurses have to wear a uniform. In British English there is a difference between them. Must is used to talk about what the speaker or listener wants, and have (got) to about rules, laws and other people’s wishes:I must finish this essay today. I’m going out tomorrow. I have to finish this essay today. We have to hand them in tomorrow. 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 a uniform? In negative sentences both must not and don’t have to are used, but with different meanings. Must not is used to tell somebody not to do something:Passengers must not smoke until the signs have been switched off. The short form mustn’t is used especially in British English: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 at weekends. note at needCertainty Both must and have to are used to say that you are certain about something. Have to is the usual verb used in North American English and this is becoming more frequent in British English in this meaning:He has (got) to be the worst actor on TV! (British English) This must be the most boring party I’ve ever been to. If you are talking about the past, use must have:Your trip must have been fun!
  3. 3  (especially British English) used to recommend that somebody does something because you think it is a good idea You simply must read this book. We must get together soon for lunch. Grammar Pointmodal verbs The 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 -ing or -ed forms 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, have to and used to:You must find a job. You ought to stop smoking. I used to smoke but I gave up 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 will find more help with how to use modal verbs at the dictionary entries for each verb.
  4. Word Origin Old English mōste, past tense of mōt ‘may’, of Germanic origin; related to Dutch moeten and German müssen.Idioms
    if you must (do something)
     
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    used to say that somebody 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
    needs must (when the Devil drives)
     
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    (saying) in certain situations it is necessary for you to do something that you do not like or enjoy
See the Oxford Advanced American Dictionary entry: must