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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.

Try this exercise to test your grammar.

Grammar test 1

Modals – deduction (past): Grammar test 1

Read the explanation to learn more.

Grammar explanation

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.

Do this exercise to test your grammar again.

Grammar test 2

Modals – deduction (past): Grammar test 2

Language level

Upper intermediate: B2

Comments

Thanks.

I am still confusing little bit about modal using past participle.

- He must do it ( present)
- He must have done it (past)

To simplify , when we mention a deduction about something happened in the past, does it always come modal verb with past principle?

Hello carmenwf.jung,

Yes, that's correct. When we are making deductions about the past we use perfect modal verbs:

must have + past participle (very sure something happened)

could/might/may have + past participle (uncertain whether something happened)

can't have + past participle (very sure something did not happen)

 

Peter

The LearnEnglish Team

It's really helpful.

Sir,

"Suspect must have left the country"
Here we feel sure about something in the past.

If I say same sentence in the future time. Here we feel sure about something in the future.
Suspect must have left the country by next morning.

Are they correct can we use this structure must have +verb 3rd form in both past and future time??

Hello Rsb,

I've answered this question on another page for you. We reply to questions as quickly as we can, though we are a small team here. If you post the same question multiple times then it only slows the process down.

 

Peter

The LearnEnglish Team

Apologize for it!

In the example, 'I might have left the air conditioning on' based on the context can represent present time meaning equivalent to a present perfect'I have left the air conditioning on'.Present perfects do have connections with the present time also.