MUST / CAN'T — Deduction
Use must and can't to make logical deductions about the present.
+She must be tired — she worked all night. (I'm almost certain this is true.)
–He can't be the manager — he only started last week. (I'm almost certain this is false.)
✏️ Form: subject + must / can't + verb (base form)
💡 This is different from the obligation use of must. Context will tell you which meaning is intended.
⚠️ Compare:
"You must wear a seatbelt." = obligation (rule)
"You must be exhausted after that journey." = deduction (logical conclusion)
SHOULD HAVE / COULD HAVE / WOULD HAVE
Use these past modal structures to talk about situations that did not happen.
+You should have called me. (You didn't call — it was a mistake.)
+I could have helped you. (I was able to help, but I didn't.)
+She would have come if she had known. (She didn't come because she didn't know.)
✏️ Form: subject + modal + have + past participle
💡 Should have = criticism or regret. Could have = unrealised possibility. Would have = hypothetical past result.
MAY
Use may for formal possibility or to give / ask for formal permission.
+There may be delays due to the weather. (possibility)
+May I see your identification, please? (formal permission)
✏️ Form: subject + may + verb (base form)
💡 May is more formal than might. Both express possibility, but may is preferred in written and professional English.
OUGHT TO
Use ought to for duty, recommendation, or expectation — similar to should but slightly more formal.
+You ought to apologise for what you said.
+The train ought to arrive at 6pm — it's usually on time.
✏️ Form: subject + ought to + verb (base form)
💡 Ought to is slightly more formal and objective than should. It often implies a moral or social duty.
Quick Summary
| Modal | Meaning | Example |
|---|
| must (deduction) | I'm almost certain it's true | She must be at home. |
| can't (deduction) | I'm almost certain it's false | He can't be serious. |
| should have | criticism / regret about the past | You should have told me. |
| could have | unrealised past possibility | I could have been hurt. |
| would have | hypothetical past result | She would have won. |
| may | formal possibility / permission | May I come in? |
| ought to | duty / recommendation | You ought to see a doctor. |