Past modal verbs of deduction

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Past Modal Verbs of Deduction

Grammar chart explaining past modal verbs of deduction: must have, can't have, may/might have, should have with examples for past speculation and criticism.

We use modal verbs + have + past participle to talk about how certain we are that something was or was not true in the past. These are called past modal verbs of deduction or speculation.

🔹Must have done

We use must have + past participle when we are quite sure something 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 / Couldn’t have done

We use can’t/couldn’t have + past participle when we are quite sure something did NOT happen 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.

❗ Important: For negative deduction, we use can’t — NOT mustn’t.
❌ He mustn’t be that famous. → ✅ He can’t be that famous.

🔹May / Might / Could have done

We use may/might/could have + past participle to say that something was possible 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.

Could have can also express a missed opportunity (something someone was able to do but didn’t).

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

❗ Note: For negative possibility, use might not or may not — NOT could not.
✅ 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 something that was expected to happen in the past — often used to express criticism or regret.

✅ 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.