The lights were off and the door was locked — they must have gone out.
You have evidence but not the facts. A past modal of deduction lets you state how sure you are about a finished action.
She didn't reply, so she may not have seen my message.
He was in Tokyo, so he couldn't have been at the party.
modal + have
+ past participle
have never changes for person, and never becomes had.
must have = it definitely happened · can't / couldn't have = it definitely didn't
The opposite of must have is can't have — not mustn't have.
may / might / could have = it possibly happened · may not / might not have = it possibly didn't
These three are interchangeable for a positive guess.
She must have missed the train.
it definitely happened
He might have forgotten the date.
it possibly happened
They may not have arrived yet.
it possibly did not happen
You can't have finished already!
it definitely did not happen
They must had left before the storm.
Correct: They must have left before the storm.
She must have went home early.
Correct: She must have gone home early.
I must of left it at home.
Correct: I must have left it at home.
He mustn't have been at the meeting.
Correct: He can't have been at the meeting.
Say the answer out loud — then tap to check.
She must haveA forgotB the meeting, becauseC she neverD arrived at the office.
Bforgot → forgotten
✓ must have + past participle
He can't haveA taken the train, becauseB the line wasC closed and the buses was fullD.
Dand the buses was full → and the buses were full
✓ the buses is plural → were
They must of leftA early, becauseB the office wasC completely empty atD four o'clock.
AThey must of left → They must have left
✓ must have, never must of
Explain the evidence with a past modal of deduction.
Never "must had", never "must of" — always "must have".