Explanations » B1+ Grammar Explanations » Past modal verbs of deduction
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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. blank
  • He can’t be that famous. blank

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.