Modals: deductions about the past

Do you know how to use modal verbs to show how certain you are about past events?

Look at these examples to see how must, might, may, could, can't and couldn't are used in the past.

• An earthquake? That must have been terrifying!

• We don't know for sure that Alex broke the coffee table. It might have been the dog.

• How did she fail that exam? She can't have studied very much.

Explanation

We can use modal verbs to talk about how sure or unsure we are about something in the past just as we use modals in the present with a slight change in the form. We use 'must have + past participle' when we are quite sure about something.

We can use modal verbs for deduction – guessing if something is true using the available information. The modal verb we choose shows how certain we are about the possibility. This page focuses on making deductions about the past.

must have

We use must have + past participle when we feel sure about what happened.

• Who told the newspapers about the prime minister's plans? It must have been someone close to him.

• The thief must have had a key. The door was locked and nothing was broken.

• Oh, good! We've got milk. Mo must have bought some yesterday.

might have / may have

We can use might have or may have + past participle when we think it's possible that something happened.

• I think I might have left the air conditioning on. Please can you check?

• Police think the suspect may have left the country using a fake passport.

May have is more formal than might have. Could have is also possible in this context but less common.

can't have / couldn't have

We use can't have and couldn't have + past participle when we think it's not possible that something happened.

• She can't have driven there. Her car keys are still here.

• I thought I saw Adnan this morning but it couldn't have been him – he's in Greece this week.