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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

Hello The LearnEnglish team,
I would like to ask whether " must have, may have, could have+ past participle" can be used to guess events or situations with present result as we use present perfect tenses. In other words, do the structures above only refer to the past or something with present result?

Hello BobMux,

Perfect forms relate a later state or event to an earlier state or event, and the perfect modal forms are no different.

These forms are used for deductive reasoning, relating present evidence or knowledge to a past situation or action:

There is blood on the floor so she must have had an accident.

present evidence: blood; past action: having an accident

 

Peter

The LearnEnglish Team

I would like to know whether we can use the passive voice in this kind context.
Would it be correct when I say "The building must have been built by the ruling coalition."

Thank you in advance for the answer!

Hi majesticdagny,

Yes! The passive is fine to describe this action/event.

Jonathan

The LearnEnglish Team

-she must've been studying hard the exam isn't easy
-they couldn't make it to dinner, they might've been sick
-he can have left already
please give me some feedback are the sentences right?

Hello wasan0909,

Aside from punctuation, the first two sentences are correct, though we would only write contractions like might've in a very informal context; generally we only contract have when used with pronouns (I, you, we etc).

 

The third sentence is not correct. We don't use can have for speculation. You could use might have, may have or could have instead.

 

Peter

The LearnEnglish Team

Hi,
Is there any difference between "can't have" and "coudn't have"?

Hi,
I really appreciate your quick response to my question. Thanks a lot.

Dear team,
Does couldn't/can't have consider as negative sentences?
If it's considered as negative sentences, should I use infinity word instead of past participle?
Thanks a lot

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