Use must when you are sure something is true and can't when you are sure it is not; for the past, use must have (done) and can't have (done).
A must and can't: what we believe is certain
Kemi lives in a flat directly above a fish and chip shop. Her friend says: It must smell of vinegar all the time.
We use must to say that we believe something is certain. We have evidence, and we have drawn the only sensible conclusion from it.
We use can't to say that we believe something is not possible.
| Subject | must / can't | Verb |
|---|---|---|
| you / she / they (etc.) | must | be (tired / hungry / at work) |
| can't | be -ing (doing / going / joking) | |
| get / know / have (etc.) |
- You've been on your feet since six this morning. You must be exhausted.
- "Gav's a brilliant cook." "Gav? You must be joking. He burns toast."
- Dylan must get lonely in that lighthouse. He sees nobody for weeks.
- You've just had a full English breakfast. You can't be hungry again.
- They only moved to Hull in March. They can't know many people yet.
B must have (done) and can't have (done): the past
Tariq and Beth have come to collect their friends for the cinema. The car is not on the drive and nobody answers the door. Beth says: They must have left without us. (= there is no other possibility)
For the past, we use must have and can't have:
| Subject | must / can't | have | Past participle |
|---|---|---|---|
| I / you / he (etc.) | must | have | been (asleep / at work) |
| can't | have | been -ing (doing / looking) | |
| gone / got / known (etc.) |
You can use couldn't have instead of can't have, with the same meaning.
- I can't find my bus pass. I must have left it on the bus. (it is the only explanation I have)
- "I grew up next to a brewery." "Did you? It must have smelt wonderful."
- Lauren hasn't replied. She can't have seen my text.
- Ryan reversed into a bollard. He can't have been looking in his mirror.
- Lauren couldn't have seen my text.
- Ryan couldn't have been looking in his mirror.
Exercises
28.1 Put in must or can't.
You've been on your feet since six this morning. You ___ be exhausted. (must / can't)
must
You've been on your feet since six this morning. You must be exhausted.
That chippy ___ be good. There's a queue down the street every Friday. (must / can't)
That chippy must be good. There's a queue down the street every Friday.
That hotel ___ be very comfortable. Every review mentions the damp. (must / can't)
That hotel can't be very comfortable. Every review mentions the damp.
I know Carl gave me his number. I ___ have it in my phone somewhere. (must / can't)
I know Carl gave me his number. I must have it in my phone somewhere.
That woman with the spaniel walks past every morning. She ___ live round here. (must / can't)
That woman with the spaniel walks past every morning. She must live round here.
Their flight was delayed for nine hours. That ___ have been much fun. (must / can't)
Their flight was delayed for nine hours. That can't have been much fun.
Congratulations on the new job. You ___ be delighted. (must / can't)
Congratulations on the new job. You must be delighted.
This gas bill ___ be right. We were away for most of the month. (must / can't)
This gas bill can't be right. We were away for most of the month.
You were only gone ten minutes. You ___ have run all the way. (must / can't)
You were only gone ten minutes. You must have run all the way.
The Hendersons go on a cruise every spring. They ___ be short of money. (must / can't)
The Hendersons go on a cruise every spring. They can't be short of money.
The lights are still on in Jo's office. She ___ be working late again. (must / can't)
The lights are still on in Jo's office. She must be working late again.
28.2 Complete each sentence with a verb (one or two words).
I can't find my bus pass. I must have left it on the bus.
I can't find my bus pass. I must have left it on the bus.
Their flat is right above a nightclub. It must be very noisy.
Their flat is right above a nightclub. It must be very noisy.
You've run the village shop for thirty years. You must everybody round here.
You've run the village shop for thirty years. You must know everybody round here.
My purse isn't in my bag. I must it in the car.
My purse isn't in my bag. I must have left it in the car.
"How old is Mr Patel?" "He was teaching when my mum was at school. He must at least sixty."
"How old is Mr Patel?" "He was teaching when my mum was at school. He must be at least sixty."
I didn't hear you come in. I must asleep.
I didn't hear you come in. I must have been asleep.
"We're off to Crete next week." "Lovely. You must forward to it." "I am."
"We're off to Crete next week." "Lovely. You must be looking forward to it." "I am."
Everybody knows that advert. You must it on the telly.
Everybody knows that advert. You must have seen it on the telly.
There are police cars all along the bypass. There must an accident.
There are police cars all along the bypass. There must have been an accident.
"Do you have a holiday home?" "You must ! I can barely pay the rent."
"Do you have a holiday home?" "You must be joking! I can barely pay the rent."
Hannah is a surgeon at the Royal Infirmary, so she must a good salary.
Hannah is a surgeon at the Royal Infirmary, so she must earn a good salary.
28.3 Use the words in brackets to write sentences with must have or can't have.
We knocked at the cottage and nobody came to the door. (they / go out)
They must have gone out.
They must have gone out.
Lauren hasn't replied to me. (she / see / my text)
She can't have seen my text.
She can't have seen my text.
Your new boots are beautifully made. (they / be / very expensive)
They must have been very expensive.
The curtains next door have been closed all week. (they / go away)
They must have gone away.
My scarf isn't on the hook. (I / leave / it at the pub last night)
I must have left it at the pub last night.
Ewan had to sell his house when the business closed. (it / be / easy for him)
It can't have been easy for him.
A woman stood at the bus stop for an hour without getting on a bus. (she / wait / for somebody)
She must have been waiting for somebody.
I asked the builder for a shed and he built a garage. (he / understand / what I wanted)
He can't have understood what I wanted.
When I got home, the back door was wide open. (I / forget / to shut it)
I must have forgotten to shut it.
There was loud music and singing from the flat upstairs until three in the morning. (they / have / a party)
They must have been having a party.
The cyclist rode straight across the zebra crossing in front of a child. (the cyclist / see / the child)
The cyclist can't have seen the child.
Grandad has had that suit since 1990, and it still looks brand new. (he / wear / it much)
He can't have worn it much.