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English Grammar in Use – Unit 28: “Must” and “Can’t”

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Part A:

Study this example:

A: My house is very near the motorway.

B: It must be very noisy.

You can use must to say that you believe something is certain:
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.)
Joe is a hard worker. Joe? You must be joking. He doesn’t do anything.’
Louise must get very bored in her job. She does the same thing every day.
I’m sure Sally gave me her address. I must have it somewhere.
You can use can’t to say that you believe 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.)
They haven’t lived here for very long. They can’t know many people.

Part B:

For the past we use must have (done) and can’t have (done).
Study this example:

There’s nobody at home. They must have gone out.

Martin and Lucy are standing at the door of their friends’ house.
They have rung the doorbell twice, but nobody has answered. Lucy says:
They must have gone out.

‘We used to live very near the motorway.’ ‘Did you? It must have been noisy.’
‘I’ve lost one of my gloves.’ ‘You must have dropped it somewhere.’
Sarah hasn’t contacted me. She can’t have got my message.
Tom walked into a wall. He can’t have been Looking where he was going.

You can use couldn’t have instead of can’t have:
Sarah couldn’t have got my message.
Tom couldn’t have been Looking where he was going.

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