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
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
Hello and thank you in advance.
I've read this sentence in a book:
“You got a letter while you were away. Sorry—I opened it. I thought it must be for me.”
I was taught that the past of “must” is “must have been” for past speculation.
Why isn’t the sentence: “Sorry—I opened it. I thought it must have been for me”?
Hello Teresa R.,
We use must have + verb 3 to talk about something that (we believe) was true in the past but is not true anymore. For example:
The sky is blue but there are puddles on the road. It must have rained last night. [it is not raining any more]
Contrast with must + verb, which describes something still true:
Look at the trees shaking. It must be very windy. [it is windy now]
The time here refers to the action being described, not our belief.
In your example the state of the letter does not change. It does not start being for you and later change. All that changes is your belief, so there is no need to use a perfect modal.
You can compare it to a non-modal form. You would not use a perfect verb here because the fact about who the letter was to did not change, so you would say I thought it was mine and not I thought it had been mine.
Peter
The LearnEnglish Team
Hello LearnEnglish Team,
We can use could have instead of may/might have when we think it's possible that something happened. But the sentence with could have + past participle can seem a conditional one:
I think might have + past participle, too, can refer to the same meaning:
It will be great help if you would explain.
Shetu Yogme.
Hello Shetu Yogme,
For past deduction, in many contexts can't have (been) and couldn't have (been) are interchangeable. Both show a logical past impossibility in the view of the speaker. However, can't have (been) tends to be used for more recent events and couldn't have (been) tends to be used for earlier events. Here is a good discussion of the topic:
https://commonenglisherrors.com/cant-have-been-couldnt-have-been/
Peter
The LearnEnglish Team
Hello again Peter,
My second question still remains unanswered.
We can use could have instead of may/might have when we think it's possible that something happened. But the sentence with could have + past participle can seem a conditional one:
I think might have + past participle, too, can refer to the same meaning:
How can we differentiate the intended meaning from the meaning originated from what we consider correct based on what we have learned?
Hello again ShetuYogme,
As you say, we can use could have to show that something was possible in the past:
Paul had the key so he could have opened the door.
We can also use could have to show a logical deduction or belief. In this use may and might are alternatives:
She didn't take my call. She could/might/may have not heard the phone, or she could/might/may have just decided she didn't want to talk.
When using could, we rely on the context to make the meaning clear. Of course, ambiguity is possible.
Peter
The LearnEnglish Team
Hi everyone,
Could you help me out with this one, please?
"I don't know why she got so many bad grades. She must'nt /can't have studied at all."
For me mustn't seems better, but is can't correct here?
Thank you.
Hi Kevcully,
Generally we use the following modal verbs for this kind of logical deduction:
These are followed by the bare infinitive for present meaning and have + verb3 for past meaning.
In some contexts, though, mustn't is also used for very unlikely conclusions. It's much less common than can't and I think is more informal but it certainly exists. I think it may also be more common in some dialects than others. Thus I would say that can't is the standard answer here but mustn't is possible.
Peter
The LearnEnglish Team
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:
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):
Peter
The LearnEnglish Team