Must and can’t 
Modal verbs of probability (deduction):
present vs past
A. Deduction about the present (now)
Use must to say you believe something is certain. Use can’t to say you believe something is not possible.
A: My house is very near the motorway.
B: It must be very noisy.
You can use must to say that you believe something is certain:
- You’ve been travelling all day. You must be tired.
- “Jim is a hard worker.” “Jim? You must be joking. He doesn’t do anything.”
- Carol must get very bored in her job. She does the same thing every day.
- I’m sure Sally gave me her phone number. 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.
- They haven’t lived here for very long. They can’t know many people.
Study the structure
B. Deduction about the past
For the past we use must have (done) and can’t have (done).
A: There’s nobody at home.
B: They must have gone out.
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.
More examples
- I didn’t hear the phone. I must have been asleep.
- “I’ve lost one of my gloves.” “You must have dropped it somewhere.”
- Sue 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
Alternative (same meaning)
You can use couldn’t have instead of can’t have:
- Sue couldn’t have got my message.
- Tom couldn’t have been looking where he was going.
Exercise 1
Put in must or can't.
1. You’ve been travelling all day. You be tired.
2. That restaurant be very good. It’s always full of people.
3. That restaurant be very good. It’s always empty.
4. I’m sure I gave you the key. You have it. Have you looked in your bag?
5. You’re going on holiday next week. You be looking forward to it.
6. It rained every day during their holiday, so they have had a very nice time.
7. Congratulations on passing your exam. You be very pleased.
8. You got here very quickly. You have walked very fast.
9. Bill and Sue always travel business class, so they be short of money.
Total Questions: 0
Incorrect Answers: 0
Exercise 2
Complete each sentence with a verb (one or two words) in the correct form.
1. I’ve lost one of my gloves. I must it somewhere.
2. They haven’t lived here for very long. They can’t 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. Sarah knows a lot about films. She must to the cinema a lot.
7. Look. James is putting on his hat and coat. He must out.
8. I left my bike outside the house last night and now it has gone. Somebody must it.
9. Amy was in a very difficult situation when she lost her job. It can’t easy for her.
10. There is a man walking behind us. He has been walking behind us for the last twenty minutes. He must us.
Total Questions: 0
Incorrect Answers: 0
Exercise 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 I / asleep
2. Sue hasn’t contacted me. She my message. get / my message
3. The jacket you bought is very good quality. It very expensive. very expensive
4. I haven’t seen the people next door for ages. They away. go away
5. I can’t find my umbrella. I it in the restaurant last night. leave / it in the restaurant last night
6. Dave, who is usually very friendly, walked past me without speaking. He me. see / me
7. There was a man standing outside the café. He for somebody. wait / for somebody
8. Liz did the opposite of what I asked her to do. She what I said. understand / what I said
9. When I got back to my car, the door was unlocked. I to lock it. forget / to lock it
10. I was woken up in the night by the noise next door. The neighbours a party. have / a party
11. The light was red, but the car didn’t stop. The driver the red light. see / the red light
Total Questions: 0
Incorrect Answers: 0