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Deduction (deciding that something is certain): must, can’t, etc

Statements: Emily must have a problem.​

Must can be used to express the deduction or conclusion that something is certain or highly probable: it is normal or logical, there are excellent reasons for believing it, or it is the only possible explanation for what is happening.

  • If A is bigger than B, and B is bigger than C, then A must be bigger than C.
  • ‘I’m in love.’ ‘You must be very happy.’
  • Emily must have a problem – she keeps crying.
  • There’s the doorbell. It must be George.

Have (got) to is also possible in this sense.

  • There’s the doorbell. It has to be George.
  • Getting married next week? You have to be joking.
  • Only five o’clock? It’s got to be later than that!

Negatives: It can’t be Emily.​

Must is not often used to express certainty in negative clauses. We normally use cannot/can’t to say that something is certainly not the case, because it is logically or practically impossible, or extremely improbable.

  • If A is bigger than B, and B is bigger than C, then C can’t be bigger than A.
  • It can’t be Emily at the door. She’s in Ireland. (not It mustn’t be Emily …)
  • She’s not answering the phone. She can’t be at home.

However, mustn’t is used in question tags ( (see here) after must, especially in British English.

  • It must be nice to be a cat, mustn’t it? (not … can’t it?)

And must not is occasionally used, especially in American English, to say that there is evidence that something is not the case (see below).

need not / does not have to​

Need not / needn’t is used (especially in British English) to say that something is not necessarily so; does not have to can also be used. Must not is not used in this sense.

  • ‘Look at those tracks. That must be a dog.’ ‘It needn’t be – it could be a fox.’ (or … It doesn’t have to be …) (not … It mustn’t be …)

That must have been nice.​

We can use must have + past participle to express certainty about the past.

  • ‘We went to Rome last month.’ ‘That must have been nice.’
  • ‘A woman phoned while you were out.’ ‘It must have been Kate.’

Can is used in questions and negatives.

  • Where can Jack have put the matches? He can’t have thrown them away.

will​

Will can express certainty or confidence about present or future situations.

  • As I’m sure you will understand, we cannot wait any longer for our order.
  • Don’t phone them now – they’ll be having dinner.
  • ‘There’s somebody coming up the stairs.’ ‘That’ll be Emily.’
  • Tomorrow will be cloudy, with some rain.

Will have + past participle refers to the past.

  • Dear Sir, You will recently have received a form …
  • We can’t go and see them now – they’ll have gone to bed.
note

For more about will as a future auxiliary, (see here).

Indirect speech​

Must can be used in past indirect speech as if it were a past tense.

  • I felt there must be something wrong.

must and should​

Should can be used as a weaker form of must (see here). Compare:

  • Anna must be at home by now. (= I’m certain she’s at home.)
  • Anna should be at home by now. (= I think she’s very probably at home.)

A British-American difference: can’t and must not​

In American English, must not is often used when something is not logically impossible, but when there is strong evidence for believing that it is not the case. Compare:

  • He only left the office five minutes ago. He can’t be home yet. (It’s logically impossible that he’s home.)
    She’s not answering the doorbell. She must not be at home. (It’s not logically impossible that she’s home, but it seems pretty certain that she isn’t.)
  • The restaurant can’t be open – the door’s locked.
    That restaurant must not be any good – it’s always empty.

In British English, can’t is normal for both meanings (though some people use must not for the ‘seems pretty certain’ meaning). Compare:

  • She walked past without saying ‘Hello’. She must not have seen you. (AmE; some British speakers.)
  • She walked past without saying ‘Hello’. She can’t have seen you. (most British speakers.)

Note that the contracted form mustn’t is rare in AmE.

must: not used for predictions​

Must is not often used for predictions about the future – to say what people expect to happen. Will is used to express certainty (see here); should or ought to suggest that things are probable (see here).

  • It will be fine tomorrow. (not It must be fine tomorrow.)
    Next week’s exam should/ought to be easy. (not Next week’s exam must be easy.)