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. |
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) |
There's a C2-level upgrade to that 50% row worth knowing about: slip "well" in right after may/might/could and the meaning shifts from "possible" to "quite likely" — "It may well rain later" is firmer than plain "it might rain." See May / Might / Could Well for the full pattern.
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)