Past modal verbs of deduction

Exercises: 1 2 3

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.

Exercises: 1 2 3

Past modal verbs of deduction: Grammar chart

A grammar chart explaining past modals of deduction, including "must," "can't," "may/might," and "should/ought to," with examples for present and past meanings.

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Modal verbs of deduction and speculation

We can use some modal verbs + an 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 mustn’t be that famous.
He can’t be that famous.

Could/might/may have done

We can use could have + past participle to say that something was possible in the past or someone had the possibility to do something but didn’t do it.

You could have called me to say you weren’t coming. I waited for hours.

We can also use could/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.

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