Modals of deduction & speculation
B1 42 / 50
In short
We use modal verbs like must, can’t, might/may/could to say how certain we are about a fact, based on the evidence we have. This is deduction (working something out), not obligation or ability.
The rule
When we don’t know something for sure, we guess from the evidence. The modal shows how strong the guess is.
Present situations (modal + base verb):
| Modal | How certain | Meaning |
|---|---|---|
| must | sure it IS true | ”The only logical answer” |
| can’t / couldn’t | sure it is NOT true | ”Impossible” |
| might / may / could | maybe true, maybe not | ”It’s possible” |
| should / ought to | expected, but not 100% | “I expect this” |
The reasoning always comes from evidence:
- The light’s on, so she must be home. (evidence -> sure yes)
- He can’t be hungry - he just ate. (evidence -> sure no)
- I’m not sure where Tom is; he might be at the gym. (open possibility)
Important: the opposite of must be (sure it’s true) is can’t be (sure it’s false), NOT mustn’t be. We do not use mustn’t for deduction.
Past situations (modal + have + past participle): we deduce about something that already happened.
| Pattern | Meaning |
|---|---|
| must have + p.p. | I’m sure it happened |
| can’t have / couldn’t have + p.p. | I’m sure it didn’t happen |
| might / may / could have + p.p. | maybe it happened |
| should have + p.p.* | I expect it has happened |
*With deduction, “they should have arrived by now” = I expect they have. Note this is different from the regret/criticism use of “should have” in lesson 26.
Forms & patterns
| Time | Sure YES | Sure NO | Possible | Expected |
|---|---|---|---|---|
| Present | must be / do | can’t / couldn’t be / do | might / may / could be / do | should / ought to be / do |
| Past | must have done | can’t / couldn’t have done | might / may / could have done | should / ought to have done |
Notes:
- The verb after the modal is always the base form (be, go, know), never to be or is.
- For the past, always use have + past participle (have gone, have known, have left).
- For ongoing actions, use a continuous form: She must be working late. / He might have been sleeping.
Examples
| English | Note |
|---|---|
| Their car’s in the drive, so they must be at home. | Evidence -> sure it’s true (present). |
| You can’t be tired already - we’ve only just started! | Sure it’s NOT true; opposite of must be, not mustn’t. |
| The keys aren’t here; I might have left them at work. | Possible, in the past: might + have + p.p. |
| Nobody’s answering. They could be asleep. | Open possibility about now. |
| She can’t have heard the news - she looks so happy. | Sure it didn’t happen; can’t + have + p.p. |
| The match started at six, so it should be finishing now. | What I expect (deduction), not certainty. |
| He must have missed the bus; he’s never this late. | Logical conclusion about the past. |
| They may not know the way - send them the map. | may not = perhaps not (not the same as can’t). |
Common mistakes
- She mustn’t be home. -> She can’t be home. - for “sure it’s false”, use can’t, never mustn’t.
- He must left already. -> He must have left already. - past deduction needs have + past participle.
- It might to rain. -> It might rain. - after a modal, use the bare base form (no to).
- They can’t be missed the train. -> They can’t have missed the train. - use have + p.p. for the past.
- You must to be joking. -> You must be joking. - no to between the modal and the verb.
Tips
- Decide your certainty first, then pick the modal: 90%+ yes = must; 90%+ no = can’t; 50/50 = might/may/could; “I expect so” = should.
- Memory hook: must and can’t are the two confident twins; might/may/could are the unsure ones.
- For anything in the past, just slot have + past participle after the same modal: must be -> must have been.
- Don’t confuse the meanings of must (lesson 26 = obligation; here = certainty) or can (lesson 17 = ability; here, can’t = impossibility).