A key fitting a lock is a clue, not a command. In a deduction, must presents a conclusion the speaker feels confident about. Might/may/could keep an explanation open; can't rejects it as incompatible with what the speaker knows.

The idea

Use a modal followed by the base verb: must be, might belong, can't know. For an activity possibly in progress, use modal + be + -ing: must be working. State the clue separately so the reader can follow the reasoning.

Examples

  • This must be the right door; our key fits the lock.
  • The studio might be empty; all the lights are off.
  • That can't be our bus; our bus is blue.
  • Lea must be working upstairs; I can hear her keyboard.
  • The delivery may be at reception.
  • This bicycle can't belong to Ben; he has never owned one.

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Watch out

Deductive must does not order anyone to act. Can't here is a negative conclusion, not inability or a ban. Mustn't commonly expresses prohibition, so it is not an automatic substitute. Confidence is the speaker's judgement, not a guarantee that the conclusion is true.

Try it yourself

  1. Choose the correct form. This must ___ the right door. (be / is)

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    be

    Modal must takes base-form be, not is.

  2. Choose the correct form. Lea must be ___ upstairs. (working / work)

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    working

    An activity inferred to be in progress uses must be working, with -ing.

  3. Express a possibility you are not sure about. The delivery ___ be at reception. (might / must)

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    might

    Might leaves the explanation uncertain. Must would express a confident conclusion, contrary to the instruction.

Use it today

Describe three clues in a room. Make one confident conclusion with must, one uncertain explanation with might and one rejected explanation with can't. Identify an alternative that could change your judgement.