Past modals

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These past modal forms are used to talk about the past in different ways. We can use them to make guesses about what happened, talk about things that were possible but didn't happen, express regrets, or talk about hypothetical past situations.

We take a modal verb, then add 'have', then add the past participle of the main verb. So the structure is: modal + have + past participle.

Must have gone.
Could have left.
Should have called.
Might have forgotten.
Can't have been.

Must have / can't have + past participle

We use these when we feel almost certain about something that happened in the past. We're making a very strong logical guess. We use 'must have' when we're almost certain that something was true.

Why didn't Julie come to meet us? She must have been at work. (This means I'm almost certain she was at work.)
The children were so quiet all afternoon. They must have been tired.
He didn't answer the phone. He must have been sleeping.

We use 'can't have' when we're almost certain something was NOT true.

She can't have gone home already — I only saw her five minutes ago.
They can't have missed the train. They left so early!
He can't have forgotten. I told him three times!

So, 'must have' is for things you're almost certain did happen, and 'can't have' is for things you're almost certain did NOT happen.

Might have / could have + past participle

We can use these to say that something was possible in the past — but we're not sure. They mean the same thing here.

She might have gone home. (This means maybe she went home, but I don't know.)
You could have left your phone on the bus. (This means maybe you left it there.)

There might have been a problem with the train.
He might have missed the flight.
She could have taken the wrong turning.
They could have forgotten about the party.

So 'might have' and 'could have' show that something was possible but you're not certain. They are much less sure than 'must have'.

(See also modals of probability.) In this case, we don't know whether what we're saying is true or not. We're just talking about our opinion of what maybe happened.

Could(n't) have + past participle

Could have + past participle means that something was possible in the past, or that you had the ability to do something, but it didn't happen or you didn't do it. (See also modals of ability.)

I could have stayed up late, but I decided to go to bed early.
They could have won the race, but they didn't try hard enough.
Julie could have bought the book, but she borrowed it from the library instead.
He could have studied harder, but he was too lazy and that's why he failed the exam.

Couldn't have + past participle means that something wasn't possible in the past, even if you had wanted to do it.

I couldn't have arrived any earlier. There was a terrible traffic jam (= it was impossible for me to have arrived any earlier).
He couldn't have passed the exam, even if he had studied harder. It's a really, really difficult exam.

Should(n't) have + past participle

1: Should have + past participle can mean something that would have been a good idea, but that you didn't do it. It's like giving advice about the past when you say it to someone else, or regretting what you did or didn't do when you're talking about yourself.

Shouldn't have + past participle means that something wasn't a good idea, but you did it anyway.

I should have studied harder! (= I didn't study very hard, and I regret it now.)
I should have gone to bed early. (= I didn't go to bed early and now I'm tired.)
I shouldn't have eaten so much cake! (= I did eat a lot of cake and now I don't feel good.)
You should have called me when you arrived. (= You didn't call me and I was worried. I wish that you had called me.)
John should have left early, then he wouldn't have missed the plane (= but he didn't leave early, and so he did miss the plane).

2: We can also use should have + past participle to talk about something that, if everything is normal and okay, we think has already happened. But we're not certain that everything is fine, so we use 'should have' and not the present perfect or past simple. It's often used with 'by now'.

His plane should have arrived by now (= if everything is fine, the plane has arrived).
John should have finished work by now (= if everything is normal, John has finished work).

We can also use this to talk about something that would have happened if everything was fine, but hasn't happened.

Lucy should have arrived by now, but she hasn't.

Would have + past participle

1: Part of the third conditional.

If I had had enough money, I would have bought a car (but I didn't have enough money, so I didn't buy a car).

2: Because 'would' (and 'will') can also be used to show if you want to do something or not (volition), we can also use would have + past participle to talk about something you wanted to do but didn't. This is very similar to the third conditional, but we don't need an 'if-clause'.

I would have gone to the party, but I was really busy.
(= I wanted to go to the party, but I didn't because I was busy. If I hadn't been so busy, I would have gone to the party.)

I would have called you, but I didn't know your number.
(= I wanted to call you but I didn't know your number, so I didn't call you.)

A: Nobody volunteered to help us with the fair.
B: I would have helped you. I didn't know you needed help.
(= If I had known that you needed help, I would have helped you.)

Continuous and passive forms

We can also use these past modal forms with continuous or passive meanings.

To add a continuous meaning, we use: modal + have + been + verb-ing.

She must have been waiting for a long time. She looked exhausted.
They might have been travelling when I called.
He can't have been working - his office was dark.
You should have been studying instead of watching television!

These sentences use the continuous form to show that the action was in progress.

We can also use the passive: modal + have + been + past participle.

The door must have been left open. That's why the cat got out.
The letter might have been sent to the wrong address.
The keys can't have been moved by accident.

Try an exercise about these past modals here.
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PAST MODALS

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