Deduction about the present
| Modal | Certainty | Example |
|---|---|---|
| must + infinitive | Almost certain (positive) | It must be cold — he's wearing a coat. She must love books — she has a huge library. |
| should + infinitive | Expectation / less certain | He should be home by now — his flight landed at 5. |
| could / might / may + infinitive | Possible (50% or less) | It might rain later — look at those clouds. She could be in a meeting. |
| can't + infinitive | Almost certain (negative) — near impossible | That can't be Tom — Tom is in Australia. It can't be a burglar — all the doors are locked. |
must vs can't:
These are opposites. Use must when you are almost certain something is true: "You must be tired." Use can't when you are almost certain something is impossible: "You can't be hungry — you just ate!"
Making a deduction about the past?
To guess about something that already happened (e.g. "she must have missed the train"), you need modal + have + past participle — that belongs to Modal Verbs in the Past, together with should have/could have for regret and criticism.
Certainty scale
| Positive | Negative | Level |
|---|---|---|
| must be | can't be | 95% certain |
| should be | shouldn't be | ~80% (expectation) |
| may be / might be / could be | might not be / may not be | ~50% (possibility) |
Exercises
Choose the modal that fits the level of certainty.
1. Keiko always does really well on exams. She ___ study a lot. (almost certain)
2. That ___ be Tom at the door — he's on holiday in Australia. (almost impossible)
3. I'm not sure, but she ___ be in a meeting right now. (possible, 50%)
4. His flight landed at 5. He ___ be home by now. (expectation, ~80%)
5. Look at those dark clouds — it ___ rain later. (possible)
6. You ___ be hungry — you just had a huge lunch! (almost impossible)