Modals: deductions about the past

Modals: deductions about the past

Do you know how to use modal verbs to show how certain you are about past events? Test what you know with interactive exercises and read the explanation to help you.

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

Average: 4.1 (78 votes)

Submitted by tunalee on Mon, 27/11/2023 - 14:57


I would like to ask the difference between can't have and couldn't have + past participle. Many thanks!

Hello tunalee,

We use both forms when we think it's not possible that something happened.

'can't have' expresses a high degree of certainty about the past. It means we think something was impossible for whatever reason, e.g. the same person being in two different places at the same time, which goes against the laws of physics as we understand them.

'couldn't have' has the idea that somebody would not have been able to do something even if they had wanted to, e.g. 'I ate way too much food. I couldn't have eaten any more.'

Other than that slight difference, i.e. the focus on wanting to do something, which isn't always even relevant, there is no difference in general.

Best wishes,
LearnEnglish team

Big thanks for your great response.
I want to ask more about the difference between can/could have + past participle and may/migh have + past participle. Sometimes it isn't very clear to use which one is better.
1. I don't know why there weren't any buses yesterday. There
............(be) a strike, but I'm really not sure.
2. The Smiths .......... (build) their house anywhere. Why did he choose here?

Thanks again for sending me useful websites.
I am grateful for your dedicated support.
I would like to ask about using could have and may/might have + past participle to make guesses about the past as mentioned on the website. What are tips to use it correctly?
It's ten o'clock. They could have arrived by now.
Where are they? They could have got lost.
I haven't received your letter. It may have got lost in the post.
It's ten o'clock. They might have arrived by now.

Hello tunalee,

The examples you give are fine but your question is rather vague. Our tips for using this structure correctly are on the page. If you have any specific questions about when to use or not use it then we will be happy to explain, of course.



The LearnEnglish Team

Thanks for your reply.
I get the examples on your website about “can and could” and “may and might”.
I am looking forward to hearing from you soon.
I would be grateful for your great and enthuasiatic answers.

Hello tunalee,

The examples sentences in our explanations are designed to illustrate the points that come just before them. Since how the grammar is being used is already explained on the page, please ask us more specific questions. We need more input from you to be able to identify what you're having a hard time understanding.

Best wishes,
LearnEnglish team

I want to ask if the difference between can/could have + past participle and may/migh have + past participle in making guesses in the past is the degrees of certainty. Or if we can depend on anything else? Thanks for your great support.