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 (80 votes)

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.

Hi tunalee,

That's not exactly it. If I say (for example) He could have left already or He might have left already or He may have left already, the meanings and degrees of certainty are roughly the same (in this case, 'I think it is possible but I'm not sure whether it actually happened'). It's not that one of them indicates more certainty than the others.

Here are a couple of other things to be aware of.

  • Could have, couldn't have and can't have are used for making deductions about the past, but can have is not.
  • The meanings of could have and couldn't have are not quite opposites. He could have left already means 'it's possible that he has left, but I'm NOT sure if it happened'. He couldn't have left already means 'I AM quite sure that he has NOT left'. In other words, they differ not just in the positive/negative content of the sentence, but the degree of certainty too.

I hope that helps.


LearnEnglish team

Submitted by Chakibon on Thu, 06/07/2023 - 11:03


I have heard some people saying " deduction in the present vs deduction in the past", which is a little bit confusing. The " deduction is always in the present. it can be about a present evidence or a past evidence.

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Submitted by Ahmed Imam on Sun, 28/05/2023 - 08:10


Hello. Could you please help me? Is the following sentence correct using "should"?
- Samar should have missed the train because she arrived at the station too late.
Thank you.

Hi Ahmed Imam,

Yes, it is correct. "Should have" can be used to show your expectation about a past event.

However, "should have" is more commonly used to express criticism (e.g. You should have studied harder before the exam) or regret (e.g. I should have studied harder), which is apparently not the meaning here. So, "Samar must have missed ..." or "Samar will have missed ..." or even "Samar has (definitely/probably) missed ..." would be my first choice of wording here.

I hope that helps.


LearnEnglish team

Yes, the sentence is correct using "should." It indicates that Samar missed the train, and the reason for her missing it was that she arrived at the station too late. The use of "should" in this context suggests an expectation or logical conclusion based on the circumstances.

Submitted by Lankarathnayake on Fri, 31/03/2023 - 08:13


I would like to give your attention on following 2 sentences.
1. We don't know for sure that Alex broke the coffee table. It might have
been the dog.
2. How did she fail that exam? She can't have studied very much.

Could you please explain can't we write the above 2 sentences as follows.

1. We don't know for sure that Alex broke the coffee table. It may be a dog.
2. How did she fail that exam? She couldn't study very much.

Thank you.

Hello Lankarathnayake,

If the second sentence in 1 is identifying the culprit, that is, the person responsible for breaking the table, you can say either 'It might have been the dog (that broke it)' or 'It might be the dog (that broke it)'. I'd say the first one is better since it refers to the person who broke the table, but the second one isn't wrong.

I'd say the first version of sentence 2 is better. We use 'wasn't able to' instead of 'couldn't' when talking about something that we managed to do in a specific situation; 'couldn't' is used to speak about general ability in the past, that is, in most situations, not specific ones such as this sentence.

Does that make sense?

All the best,
LearnEnglish team

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

This sentence makes perfect sense. It expresses the idea the the speaker is certain that the subject did not study much. She's surprised, and then she makes a deduction to explain her surprise.