MUST / CAN'T — Present Deduction
Use must when you are almost certain something is true. Use can't when you are almost certain it is not true.
+You've been working since six — you must be exhausted.
–That can't be her coat — she never wears red.
✏️ Form: subject + must / can't + base verb
💡 These are logical deductions — the speaker has evidence and draws a conclusion.
SHOULD / COULD / WOULD HAVE — Past Modals
Use past modals to talk about what was expected, possible, or hypothetical in the past.
+You should have told me earlier — I could have helped.
+She could have taken the earlier train but she missed it.
+I would have joined you, but I had a prior commitment.
✏️ Form: subject + modal + have + past participle
💡 Should have = criticism or regret. Could have = unrealised past possibility. Would have = hypothetical past result.
MAY
Use may for formal permission or to express a possibility more tentatively than might.
+The board may reconsider the decision at the next meeting.
+You may leave the room once you have finished the assessment.
💡 May is slightly more formal than might and is common in written and official English.
OUGHT TO
Use ought to similarly to should — for moral obligations, advice, and expectations.
+You ought to get that checked by a doctor.
+The results ought to be ready by now — I'll chase them up.
✏️ Form: subject + ought to + base verb
💡 Ought to is slightly more formal than should. It implies a moral or practical expectation.
Quick Summary
| Modal | Meaning | Example |
|---|
| must | strong deduction (almost certain it's true) | She must be tired. |
| can't | strong deduction (almost certain it's false) | That can't be right. |
| should have | criticism / regret about past | You should have checked. |
| could have | unrealised past possibility | It could have been worse. |
| would have | hypothetical past result | I would have told you. |
| may | formal possibility / permission | You may go now. |
| ought to | moral expectation / advice | You ought to apologise. |