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)
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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.

Hello andyroo,

Yes, you are absolutely right. I'm not sure what I was thinking! I've amended my previous comment so that no one gets confused.

Thanks for taking the time to point this out to us!

All the best,
LearnEnglish team

Submitted by DANIELEBRAUER on Thu, 23/03/2023 - 20:41


Hi, everyone! I am in desperate need for help!

I keep finding different information when it comes to negative past modals of deduction:
Some materials will say that it is possible to say:
"I must not have seen her" as well as
"I couldn't have seen her"
Others say that the only possibility is "couldn't have" and that "mustn't have" is never used as a modal of deduction. I'm completely lost!!!!!


I (native English speaker from the north of England) would not use mustn't have in this kind of context but that's not to say that it does not occur in some dialects of English.


I do sometimes use 'must have' with a negated past participle. For example:

A: How is it possible that you were both at the party and you didn't give her my message?

B: I'm really sorry. I must have not seen her.


Note there is a difference in meaning here between couldn't have seen and must have not seen

> couldn't have seen - this action is not logically possible

> must have not seen - this is the only possible explanation

Obviously, in the context of justifying myself the second is more useful!



The LearnEnglish Team

Submitted by howtosay_ on Tue, 17/01/2023 - 14:25



Could you please say if it is correct "You must have me confused with other person" or am I to say "You must have confused me with other person"?

I'm grateful for your helpful work and thank you very much for answering this post beforehand!

Hi howtosay_,

No worries. Thanks for posting your questions :)

Both of these are grammatically correct. The first sentence (have + object + confused with something) refers to now - the person is confused right now.

The second sentence (have confused + object + with something) is the present perfect and it refers to a confusion that has already happened.

It should also be "another person" (instead of "other person"). Hope it helps!


The LearnEnglish Team

Submitted by wilson3827 on Sat, 14/01/2023 - 18:23


Hello, I'm wondering if what I wrote is correct:

"The referee could have disallowed the goal and the same controversy would have remained."

Context: there was a controversial goal. Some people said it was a goal, while others said it shouldn't have stood. I think that in case the referee had disallowed the goal the controversy would have remained.

My problem: I feel my phrase is not completely right because it combines two past-modal verbs "could have" + "would have". Maybe this is not a right structure.

Thanks in advance.