Grammar · B2
Must have, might have, should have, needn’t have
Modal perfects let you talk about the past with a degree of certainty attached. Must have is confident deduction, can’t have is confident denial, might have leaves the question open, should have is reproach or regret, and needn’t have means you did something that turned out to be unnecessary.
A modal perfect is a modal verb plus have plus the past participle. It is how English comments on the past rather than simply reporting it.
For deduction, the scale runs must have (I am sure it happened), might or could have (it is possible), and can’t or couldn’t have (I am sure it did not).
For evaluation, should have and ought to have express reproach or regret: you should have told me.
Two forms are regularly confused. Needn’t have done means you did it and it turned out to be unnecessary. Didn’t need to do usually means it was not necessary, so you did not do it.
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He is not answering and his coat has gone. He left already.