must/can't/may/might/could have done · should/could/might have done · needn't have
10.1 Modal auxiliary verbs 2 — probability in the past
All modal auxiliary verbs can be used with the perfect infinitive. They express varying degrees of certainty. Will have done is the most certain, and might/could have done is the least certain.
| Modal + have + PP | Certainty | Example |
|---|
| will have been | Most certain | 'I met a girl at your party. Tall. Attractive.' 'That'll have been Sonya.' |
| must have been | Logical deduction | It must have been a good party. Everyone stayed till dawn. |
| can't have | Logically impossible | The music can't have still been at dawn. Nobody danced. |
| may have got | Possible | He may have got lost. |
| might have decided | Possible | He might have decided not to come. |
| could have had | Less definite | He could have had an accident. |
Would have thought is common to express an assumption or supposition.
I'd have thought they'd be here by now. Where are they?Other uses of modal verbs in the past
should have done
- Should have done can express advice or criticism about a past event — the sentence expresses what is contrary to the facts: You should have listened to my advice. (You didn't listen.)
- Should have done can be used for comic effect when the suggestion is 'because it was so funny': You should have been here yesterday! You should have seen his face!
could have done
- Could have done is used to express an unrealized past ability — someone was able to do something in the past, but didn't do it: I could have gone to university, but I didn't want to.
- It is also used to express a past possibility that didn't happen: When I took the burnt meal out of the oven, I could have cried!
- It is used to criticize people for not doing things: You could have told me that Sue and Jim had split up! You could at least have done your bedroom!
might have done
- The above use of could have done can also be expressed with might have done: You might have helped instead of just sitting on your backside!
- I might have known/guessed that … is used to introduce a typical (usually annoying) action of someone or something.
needn't have
Needn't have expresses an action that was done, but wasn't necessary. It was a waste of time.
I needn't have got up so early. The train was delayed.