must: deduction

  • We can use must lo say that we are sure about something (because it is logically necessary).
      If A is bigger than B, and B is bigger than C, then A must be bigger than C.
      Mary keeps crying. She must have some problem.
      There’s the doorbell. It must be Roger.
      ‘I’m in love. ‘ ‘That must be nice.’
  • In questions and negatives, we use can and can’t with this meaning, not must and mustn’t.
      ‘There’s somebody at the door. Who can it be?
      It can’t be the postman. It’s only seven o’clock.’
      What do you think this letter can mean?
  • We use [must have + past participle] for deductions about the past
      (can have in questions and negatives).
      [must/can/can’t have + past participle]
      ‘We went to Rome last month.’ ‘That must have been nice.’
      I don’t think he can have heard you. Call again.
      Where can John have put the matches? He can’t have thrown them away.