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!"
Deduction about the past
To make a deduction about something that already happened, add have + past participle after the modal.
| Modal | Certainty | Example |
|---|---|---|
| must have + pp | Almost certain (positive) | Susie is so late! She must have missed the train. Her husband must have bought those flowers. |
| might/may/could have + pp | Possible | She might have left her keys at the office. |
| can't have + pp | Almost certain (negative) | He can't have eaten all of that — it was a huge pizza! |
There's rubbish all over my garden! A fox must have been in the bin.
The kitchen was spotless. Lily must have tidied it before she went to bed.
The kitchen was spotless. Lily must have tidied it before she went to bed.
Certainty scale
| Positive | Negative | Level |
|---|---|---|
| must be / must have done | can't be / can't have done | 95% certain |
| should be | shouldn't be | ~80% (expectation) |
| may be / might be / could be | might not be / may not be | ~50% (possibility) |
Exercises
must + infinitive OR must + have + past participle?
1. Keiko always does really well on exams. She ___ (study) a lot.
2. Susie is so late! She ___ (miss) the train.
3. Where is my purse? I saw it earlier, so it ___ (be) in this room.
4. There were flowers on the table when she got home. Her husband ___ (buy) them.
5. Oh no, I don't have my keys! I ___ (leave) them in the taxi.
6. If you haven't eaten all day, you ___ (be) hungry.