Certainty: must/can't/couldn't

MUST/CAN'T/COULDN'T are used to express certainty about a present event.

MUST		BE/DO (Present Infinitive)/ BE ... -ING (Present Continuous Infinitive)
CAN’T	+
COULDN’T

MUST is used to say that we are sure about something the present.

There's a lot of noise from upstairs. It must be Tom. I meet her rather often. She must be living nearby.

CAN’T (not MUSTN'T/COULDN^’ T) are used to say that we are sure that something is impossible.

Can I have some sweets? I'm hungry. You can't/couldn't be hungry. You've just had dinner!

Exercise 22. Rewrite each sentence so that it contains must, can't or couldn't.

Example: I'm sure that Jack feels really lonely. Jack must feel really lonely. 1. I'm sure she isn't in bed. She can't/couldn't be in bed.

2. I'm sure that Helen is over twenty.

3. I'm sure he's not feeling very well.

4. I'm sure she is playing tennis now.

5. I'm sure it's not nine o'clock.

6. I'm sure that he is not the thief.

MUST/CAN’T/COULDN'T are used to express certainty about a past event.

MUST		HAVE BEEN (Perfect Infinitive)/ HAVE BEEN DOING (Perfect Continuous Infinitive)
CAN’T	+
COULDN’T

He must have taken sleeping pills last night. He didn't wake up till lunch time.

You can't/couldn't have taken the wrong road.

— I waited under the clock!

— So did I, but I didn't see you! We must have been waiting under different clocks.

Exercise 23. Rewrite the sentences. Use must/can't/ couldn't to express certainty about a past event.

Example: Obviously it was a long journey. It must have been a long journey.

1. I'm sure she's passed her driving test.

2. Certainly she saw us.

3. I'm sure you've heard the news.

4. I'm sure the bus has left.

5. Obviously he was waiting for you outside the cinema.

Exercise 24. Complete each sentence so that it contains must/can't/couldn't.

1. The cat be somewhere in the house.

2. Tom have been at the theatre yesterday! I know he is in hospital.

3. They be waiting for Jane at the airport. It's half past six now and her plane arrives at 7 p.m.

4. Annie be asleep. There's a light on in her bedroom.

5. Those shoes you bought are very nice. They have been expensive.

6. You've been working very hard all day. You be feeling very tired.

Exercise 25. Choose the best answer A or B.

1. There's someone at the door. It the postman.

A) can be; B) must be.

2. Jones president if Smith has to resign.

A) could be; B) must be.

3. Let's tell Diana. She know.

A) could not; B) might not.

4. In my opinion, the government something about this.

A) might do; B) should do.

5. — Why did you worry about me? I didn't take any risk. — You .

A) must have been injured; B) could have been injured.

6. Do you think we ask before we borrow the car?

A) ought to; B) could.

7. Don't stand in the boat! You fall in the river.

A) ought to; B) might.

8. Me learn to drive! You be joking!

A) must; B) should.

9. That's a stupid idea! You be serious, surely!

A) might not; B) cannot.

10. What I do in this awkward situation? Do you have any idea?

A) shall; B) could.

11. Why is the dog barking? It heard something.

A) can't have; B) must have.

12. You smoke at your age.

A) shouldn't; B) shan't.