Study this example:
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.) must/can't be (tired/hungry/at work etc.)
I/you/he (etc.) must/can't be (doing/coming/joking etc.) do/go/know/have etc.
I/you/he (etc.) must/can't 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.) must/can't have been (asleep/at work etc.)
I/you/he (etc.) must/can't have been (doing/working etc.)
I/you/he (etc.) must/can't have done /gone/known/had etc.
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.
EXERCISES
28.1 Put in must or can't.
1. You've been travelling all day. You must be very tired.
2. That restaurant --- be very good. It's always full of people.
3. That restaurant --- be very good. It's always empty.
4. You're going on holiday next week. You --- be looking forward to it.
5. It rained every day during their holiday, so they --- have had a very nice time.
6. Congratulations on passing your exam. You --- be very pleased.
7. You got here very quickly. You --- have walked very fast.
8. Bill and Sue go away on holiday very often, so they --- be short of money.
28.2 Complete the sentences with a verb in the correct form.
1. I've lost one of my gloves. I must have dropped it somewhere.
2. They haven't lived here for long. They can't know many people.
3. Ted isn't at work today. He must --- ill.
4. Ted wasn't at work last week. He must --- ill.
5. (The doorbell rings) I wonder who that is. It can't --- Mary. She's still at work at this time.
6. Carol knows a lot about films. She must --- to the cinema a lot.
7. Look. Jack is putting on his hat and coat. He must --- out.
8. I left my bike outside the house last night and this morning it isn't there any more. Somebody must --- it.
9. Ann was in a very difficult situation. It can't --- easy for her.
10. There is a man walking behind us. He has been walking behind us for the last 20 minutes. He must --- us.
28.3 Read the situations and use the words in brackets to write sentences with must have and can't have.
1. The phone rang but I didn't hear it. (I/asleep)
_I must have been asleep._
2. Jane walked past me without speaking. (she/see/me)
_She can't have seen me._
3. The jacket you bought is very good quality. (it/very expensive)
4. I haven't seen the people next door for ages. (they/go away)
5. I can't find my umbrella. (I/leave/it in the restaurant last night)
6. Don passed the exam without studying for it. (the exam/very difficult)
7. She knew everything about our plans. (she/listen/to our conversation)
8. Fiona did the opposite of what I asked her to do. (she/understand/what I said)
9. When I woke up this morning, the light was on. (I/forget/to turn it off)
10. The lights were red but the car didn't stop. (the driver I see/the red light)
11. I was woken up in the middle of the night by the noise next door. (the neighbours/have/a party)
28.1
2 must 3 can't
4 must 5 can't
6 must 7 must 8 can't
28.2
3 be
4 have been
5 be
6 go or have been
7 be going
8 have taken/have stolen
9 have been
10 be following
28.3
3 It must have been very expensive.
4 They must have gone away.
5 I must have left it in the restaurant last night.
6 The exam can't have been very difficult.
7 She must have been listening to our conversation./She must have listened to.
8 She can't have understood what I said.
9 I must have forgotten to turn it off.
10 The driver can't have seen the red light.
11 The neighbours must have been having a party.
