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:
- Sarah couldn't have got my message.
- Tom couldn't have been looking where he was going.
