English — Modals of past deduction
To make deductions about the past, use a modal verb + have + past participle. The modal you choose shows how sure you are: must have signals near-certainty, can't have and couldn't have rule something out, and might have, may have and could have leave room for doubt.
How it works
Use must have + past participle when the evidence makes a conclusion almost certain: The ground is wet — it must have rained. Use can't have or couldn't have + past participle to reject a conclusion: She can't have finished so quickly. Use might have, may have or could have + past participle when something is only possible: He might have taken the earlier train.
Forms and meanings
| Form | Example | Meaning |
|---|---|---|
| must have + past participle | She must have left already. | Near-certain (positive) |
| can't have + past participle | He can't have seen us. | Ruled out (certain negative) |
| couldn't have + past participle | They couldn't have known. | Impossible |
| might have + past participle | It might have rained overnight. | Possible |
| may have + past participle | She may have forgotten the meeting. | Possible |
| could have + past participle | He could have missed the train. | Possible |
Good to know
Don't confuse could have (a real past possibility) with can't have (something ruled out). Also avoid the common spoken mistake must of — the written form is always must have, often reduced to must've in speech.