Past modal verbs of deduction

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.