MUST / CAN'T — Present Deduction
Use must when evidence leads you to a near-certain conclusion. Use can't when the evidence makes something seem impossible.
+Her office light is still on — she must still be working.
–This can't be the right room — there's nobody here.
💡 Both are logical deductions — the speaker is not guessing randomly but drawing a conclusion from visible evidence.
SHOULD / COULD / WOULD HAVE
These past modal structures refer to what was expected, possible, or hypothetical in the past.
+They should have issued a warning before the deadline passed.
+We could have resolved this weeks ago with one conversation.
+She would have accepted the offer if the timing had been better.
⚠️ Should have = it didn't happen but it should have (criticism/regret).
Could have = it was possible but didn't happen.
Would have = it would have happened under different conditions.
MAY
Use may for formal possibilities or to grant permission in a formal register.
+The findings may indicate a wider structural problem.
+You may use a dictionary during the written paper.
💡 May is more formal than might and is common in academic writing, official notices, and formal speech.
OUGHT TO
Use ought to for moral expectations, strong recommendations, or when something is logically expected.
+He ought to have replied by now — it's been three days.
+Companies ought to be more transparent about their data practices.
✏️ Form: subject + ought to + base verb
💡 Ought to have + past participle = something was expected but did not happen.
Quick Summary
| Modal | Meaning | Example |
|---|
| must | deduction — almost certainly true | She must know. |
| can't | deduction — almost certainly false | That can't be right. |
| should have | criticism / regret | They should have warned us. |
| could have | unrealised past possibility | We could have avoided it. |
| would have | hypothetical past result | I would have helped. |
| may | formal possibility / permission | Results may vary. |
| ought to | moral expectation / recommendation | He ought to apologise. |