MUST / CAN'T — Deduction
Use must when you conclude something is almost certainly true. Use can't when you conclude it is almost certainly false.
+The keys are here — he must have come back.
–She can't have finished already — it was a three-hour test.
💡 Must have + past participle = past deduction (almost certainly happened).
Can't have + past participle = past negative deduction (almost certainly didn't happen).
SHOULD / COULD / WOULD HAVE
Past modals express what was expected, possible, or hypothetical before a past moment.
+The report should have been proofread before it was sent.
+He could have asked for an extension — the policy allows it.
+I would have attended if I hadn't been travelling that week.
💡 Should have = it didn't happen and it should have.
Could have = it was possible but didn't happen.
Would have = it would have happened under different conditions.
MAY
Use may to express formal possibility or to grant permission in a formal register.
+The committee may choose to defer the vote until further evidence is available.
+Participants may request a copy of their individual results.
💡 In formal and written contexts, may is preferred over might for possibility.
OUGHT TO
Use ought to for moral obligations, strong recommendations, and logical expectations.
+The council ought to publish its findings within thirty days.
+You ought to have been given more warning — that wasn't fair.
✏️ Ought to have + past participle: something was expected or should have happened but didn't.
Quick Summary
| Modal | Meaning | Example |
|---|
| must / must have | deduction — almost certainly true | She must have left. |
| can't / can't have | deduction — almost certainly false | He can't have seen it. |
| should have | criticism / regret | It should have been checked. |
| could have | unrealised past possibility | He could have asked. |
| would have | hypothetical past result | I would have gone. |
| may | formal possibility / permission | Results may differ. |
| ought to | moral expectation / strong advice | You ought to reconsider. |