Study materials
You come into the office in the morning and find the door unlocked and the lights on. You do not know what happened overnight, but you draw a conclusion at once.
Someone must have stayed late.
They can't have locked the door.
It is the same scale of certainty as for the present, only turned towards the past. Only one detail of the form changes.
One formula for the whole topic
The pattern is always the same: a modal, then have, then the third form of the verb. The modal never changes, and have never becomes has, not even for he or she.
She must have forgotten.
He should have called.
In speech the have shrinks almost to nothing: must've, should've. By ear it merges with the word before, which is exactly why so many people write the mistaken must of.
The scale of certainty
The first and commonest use is a conclusion about something you did not see. The words line up from near certainty to logical impossibility.
| Form | How sure you are |
|---|---|
| must have done | almost certain it was so |
| may, might, could have done | perhaps, perhaps not |
| can't, couldn't have done | almost certain it was not |
The lights were off. They must have left early.
She might have missed the train.
He can't have known about it.
Remember the pair: must have for a positive conclusion, can't have for a negative one. The form mustn't have is not used in this sense at all.
The mistakes everyone makes
The first is writing of instead of have.
She must of forgotten. → She must have forgotten.
The second is taking mustn't have for a negative conclusion.
He mustn't have known. → He can't have known.
The third is changing have into has.
She must has left. → She must have left.
All three are caught by one look at the chain. A modal, have, the third form, and no changes in any of the three parts.
Try it right now
Think of a time when somebody did not turn up or did not answer, and build three guesses in English: one confident, one uncertain, and one about what definitely did not happen.
Check that the last one uses can't have.