Modals of deduction in the past
Guessing about the past from evidence: she must have forgotten, he can't have seen it, they might have missed it.
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At B1 you used must, might and can't to guess about the present: She must be at home. For a guess about the past, use the same modal verbs + have + past participle. How sure you are decides the modal.
How sure are you?
How sure | Modal + have + past participle |
|---|---|
sure it happened | must have gone |
it possibly happened | might / may / could have gone |
sure it didn't happen | can't / couldn't have gone |
- The streets are wet. It must have rained in the night.
- I can't find my keys. I might have left them at work.
- She can't have seen us. She didn't say hello.
The form: modal + have + past participle
The modal never changes, have never changes, and the main verb is the past participle (gone, seen, forgotten, been).
- Incorrect: He must went home early.
- Correct: He must have gone home early.For the past, use must have + past participle, not must + a past form.
- Incorrect: She must have forget about the meeting.
- Correct: She must have forgotten about the meeting.After have, use the past participle: forgotten.
In speech, must have sounds like "must've" /ˈmʌstəv/. Write must have or must've, never "must of".
Must have: I'm sure it happened
Use must have when the evidence makes you almost certain.
- You must have been tired after such a long flight.
- The lights are off. They must have gone to bed.
- "I got the job!" "You must have made a great impression."
Can't have and couldn't have: I'm sure it didn't happen
The opposite of must have is can't have or couldn't have.
- He can't have seen the email. He was on a plane all day.
- You couldn't have met my grandfather. He died before you were born.
- Incorrect: They can't be at home last night. Their car wasn't there.
- Correct: They can't have been at home last night. Their car wasn't there.The guess is about last night, so use can't have + past participle.
In British English, the usual negative is can't have. In American English you will also hear must not have: He must not have heard me.
Might have, may have, could have: perhaps it happened
When you are not sure, use might have, may have or could have. The negative is might not have or may not have.
- She might have missed the bus.
- They may have gone to the cinema.
- Someone could have taken it by mistake.
- He might not have heard the phone.
- Incorrect: I'm not sure, but he couldn't have heard me.
- Correct: I'm not sure, but he might not have heard me.Couldn't have means you are sure it didn't happen. If you aren't sure, use might not have.
Could have also means that something was possible but didn't happen: Be careful! You could have hurt yourself. The situation shows which meaning it has.
Must have been doing: an activity in progress
For an activity that was in progress at a past moment, use the modal + have been + -ing.
- He didn't answer. He must have been sleeping.
- They might have been joking. I couldn't tell.
- She can't have been driving. Her car was at home.
Present or past?
The same modal can look at now or at the past. Compare:
- She must be at work. (I think so now.)
- She must have been at work. (I think so about yesterday.)
- He might be asleep. / He might have been asleep.
Must have can also look at a finished result up to now: They must have arrived by now.
Questions
In questions, use could have or might have, or Do you think ...?
- Could she have forgotten?
- Where might they have gone?
- Do you think he took the wrong train?
For criticism of the past (You should have told me), see the page on should have and needn't have.