Modal verbs of deduction: Must, may, might, could, can’t

Modal verbs of deduction: Must, may, might, could, can’t

Exercise 1

Choose all the correct modal verbs of deduction for each gap below. In some sentences there are TWO possible correct answers.

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1 Paul is behaving in a very unusual way. I think he ______ again.

A.can't drink

B.can be drinking

C.might be drinking

2 I think there ______ a mistake in your tax return. You should check it. Choose TWO correct options

A.can't be

B.might be

C.may be

3 If Suzan said that, it ______ true. She never lies.

A.might be

B.must be

C.can be

4 Sorry, but I'm not Connor. You ______ me for someone else.

A.might be confusing

B.must confuse

C.must be confusing

5 I'm not sure I trust Peter. He ______ the person we think he is.

A.might not be

B.can't be

C.must not be

6 You have been walking for ten hours. You ______ exhausted.

A.can be

B.can't be

C.must be

7 He ______ his son, they look completely different.

A.might be

B.can't be

C.must not be

8 There's a bit of traffic, so I ______ arrive in time. Choose TWO correct options

A.might not

B.must not

C.may not

9 He ______ be very proud of you right now. You disappointed him.

A.must not

B.can't

C.might

10 I wouldn't swim in this river if I were you. It ______ dangerous. Choose TWO correct options

A.might be

B.may be

C.can be

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Modal verbs of deduction: Grammar chart

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May, might

We use may and might to talk about things that are possibly true, but we don’t know for sure.

• He’s more than 2 meters tall. He might be a basketball player. (=perhaps he is)
• He says Betty is his friend, but I think she may be his girlfriend.

We use may not or might not to talk about things that are possibly not true, but we don’t know for sure.

• You should call her. She might not know where you are. (=perhaps she doesn’t know)

Don’t use can for deduction

We don’t use can as a modal of deduction.

• He can be at home now.
• He might/may be at home now.

Must, can’t

We use must when we are sure, or quite sure, that something is true.

• You must be tired after the long journey. (=I’m sure you are tired)
• I’m sure I had the keys when I left. They must be in the car.

❗ But we use can’t (NOT mustn’t) when we are sure, or quite sure, that something is not true.

• We’ve been walking for hours. It mustn’t be far from here.
• We’ve been walking for hours. It can’t be far from here. 
• They’ve lived here only for a couple of months. They can’t know many people.

+ be + -ing

After may, might, must or can’t, we can use be + -ing, when we are talking about actions in progress.

• They’ve gone to Ibiza, and right now, they must be having a great time.
• Call him. He might be waiting for us.