Luyện ngữ pháp - English Grammar in Use - Intermediate ((Unit 28: Must and can’t))

Grammar Explanations and Rules

Must and can’t

Study this example:

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.

Study the structure:

I/you/he (etc.)

must
can't

be (tired I hungry I at work etc.)
be (doing / going / joking etc.)
do / get / know / have etc.

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

Study this example:

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.

Study the structure:

I/you/he (etc.)

must
can't

have

been (asleep / at work etc.)
been (doing / looking etc.)
gone / got / known etc.

You can use couldn't have instead of can't have: