Exercise 3

Complete the sentences using the verbs in brackets with a past modal verb form.

Old Mr McGraw had been murdered and everybody was wondering who 1 (do) it. Two police officers were speculating about the murder.

SEBASTIAN: It 2  (be) Mrs McGraw. I’m totally convinced it was her. She absolutely hated him. She 3  (use) a kitchen knife as the murder weapon, or maybe an axe from the garden tools.

JESSICA: It 4  (be) Mrs McGraw. She isn’t strong enough to drag the body and put it in the car boot. It 5  (be) their older son; I would bet all my money it was him. He had motive, because Mr McGraw had disinherited him long ago, and everybody knew they hated each other.

SEBASTIAN: Impossible. It 6 (be) their older son because he has a plausible alibi.  He was fishing with his brother at the time of the crime, he testified.

JESSICA: Then the younger son 7 (lie) to protect his brother. They 8  (agree) on an alibi after the murder. If you think about it, they 9  (plan) the murder together.  It’s a reasonable possibility. Maybe the younger son was afraid that his father would disinherit him too; they had a difficult relationship.

SEBASTIAN: That makes sense. They killed him so that at least one of them could get the inheritance.

JESSICA: Well, if they wanted to get the money from the inheritance, they 10  (kill) their father. Because now we’ll catch them and they will be in prison and without a penny.



Past modal verbs of deduction – grammar chart

Past modal verbs of deduction

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Modal verbs of deduction and speculation

We can use some modal verbs + infinitive to talk about how certain we are that something is or is not true. We can also use some modal verbs + have + past participle to talk about how certain we are that something was or was not true in the past.

must have done

We use must have + past participle to say that we are quite sure that something was true or happened in the past.

  • I can’t find my wallet! I must have dropped it in the taxi.
  • You must have had a real scare when you saw the crocodile. 


can’t have done

We use can’t/couldn’t have + past participle to say that we are quite sure that something did NOT happen or was NOT true in the past.

  • You can’t/couldn’t have seen John last night. He was in hospital. 
  • She can’t/couldn’t have passed the test. She didn’t even open the books. 

Note that for negative deduction we use can’t (NOT mustn’t)

  • He can’t be that famous. (NOT He mustn’t be that famous.)


could/might/may have done

We use might/may have + past participle to say that it’s possible that something was true or happened in the past.

  • It’s been three days. They might/may have finished painting the house by now. 
  • If they left at 9, they might/may have already arrived.

Note that we use might not or may not (NOT could not) to talk about a negative possibility.

  • She might/may not have heard us. Knock again. 


should/ought to have done

We use should have/ought to have + past participle to talk about a situation expected to happen in the past. This form is normally used for criticism.

  • You should/ought to have already packed your things. We’re going to be late. 
  • He should/ought to have studied more. Nobody fails if they study.