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 (84 votes)

Submitted by riverolorena67 on Sat, 15/06/2024 - 21:47


This was a famous case about a prosecutor who had been murdered. 

The fatal shot was fired from a gun that had been loaned to Nisman by aide Diego Lagomarsino, a computer technician who said the prosecutor asked for the weapon because he feared for his life and his daughters’ lives.

Is it right to use MIGHT HAVE KILLED for a possibility in the past?

Diego Lagomarsino  lent Nisman the weapon  because he feared for his life and his daughters’ lives.

Nisman was worried that somebody....MIGHT HAVE KILLED (kill) him or his daughters.

Hello riverolorena67,

Might have is used when the speaker does not know if the action was done or not. For example:

Paul might have arrived already. Let's check.

The speaker does not know if Paul has arrived.


In your example, the speaker knows the true situation because he and his daughters were alive at the time of the request (asking for the gun). The best choice here is simply to use might + verb (not a perfect form):

Nisman was worried that somebody might (try to) kill him or his daughters.



The LearnEnglish Team

Profile picture for user Ahmed Imam

Submitted by Ahmed Imam on Tue, 21/05/2024 - 18:22


Hello Team. In an exam today, we had the following sentence. Could you please help me choose? 

- Salah (can't - mustn't) have played yesterday’s match. He was injured.

We are confused! Simple language please.

Thank you.

Hello Ahmed Imam,

The correct answer here is can't.

For deductions in the past there are five modal verbs commonly used:

  • strong belief: must have + verb3
  • uncertain but possible: could have / might have / may have + verb3
  • strong disbelief: can't have + verb3

Thus the opposite of must have played (strong belief) is can't have played, not mustn't have played.



The LearnEnglish Team

Submitted by ShetuYogme on Tue, 14/05/2024 - 08:54


Hello learn English team,

Can we use "will have+past participle" to refer to the past deduction? For example:

1. He will have reached the city by now.

2. You will have enjoyed the party yesterday.

If yes, then what is the difference between ''will have+ past participle" and "must have + past participle"?

1. He will have reached the city by now.

He must have reached the city by now.

2. You will have enjoyed the party yesterday.

You must have enjoyed the party yesterday.

I have often seen people use "would have" when discussing past deductions. Is it possible to use would have to past deductions? Examples:

1. Everyone would have enjoyed the party.

2. You would have felt lonely without me.

I am confused about this topic. Would you please help me understand this complex topic? It would be very helpful for me if you could please explain my queries in detail and provide some references to understand this topic comprehensively.

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?