Must and can't

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Must and can't

A. Study this example:

  • A: My house is very near the motorway.
  • B: It must be very noisy.

We use must to say that we feel sure something is true:

  • You've been travelling all day. You must be tired. (Travelling is tiring and you've been travelling all day, so you must be tired.)
  • 'Jim is a hard worker.' 'Jim? A hard worker? You must be joking. He's very lazy.'
  • Carol must get very bored in her job. She does the same thing every day.

We use can't to say that we feel sure something is not possible:

  • You've just had lunch. You can't be hungry already. (People are not normally hungry just after eating a meal. You've just eaten, so you can't be hungry.)
  • Brian said he would definitely be here before 9.30. It's 10 o'clock now and he's never late. He can't be coming.
  • They haven't lived here for very long. They can't know many people.

Study the structure:

 I/you/he (etc.)  

be (tired / hungry / at work etc.)
be (doing / coming / joking etc.)
do / go / know / have etc.

B. For the past we use must have (done) and can't have (done).

Study this example:
George is outside his friends' house. He has rung the doorbell three times but nobody has answered.
They must have gone out. (otherwise they would have answered)

  • The phone rang but I didn't hear it. I must have been asleep.
  • I've lost one of my gloves. I must have dropped it somewhere.
  • Jane walked past me without speaking. She can't have seen me.
  • Tom walked straight into a wall. He can't have been looking where he was going.

Study the structure:

 I/you/he (etc.)  

been (asleep / at work etc.)
been (doing / working etc.)
done / gone / known / had

Couldn't have... is possible instead of can't have:

  • She couldn't have seen me.
  • Tom couldn't have been looking where he was going.


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