Home › B1 › Grammar › Must, might, could, can’t for deduction (present)
Must, might, could, can’t for deduction (present)
B1 B1 grammar · lesson 11 of 23
We often guess about situations using the information we have. Modal verbs show how sure we are, from almost certain to just possible.
How sure are you?
When we don’t know something for a fact, we can make a deduction (a logical guess). We use a modal verb + infinitive without to.
| How sure? | Modal | Example |
|---|---|---|
| almost 100% sure it’s true | must | She must be at home. The lights are on. |
| possible (maybe 50%) | might / may / could | He might be in the garden. |
| almost 100% sure it’s not true | can’t | That can’t be Tom. He’s in Paris. |
The opposite of must in this meaning is can’t, not mustn’t. For «maybe not», use might not or may not.
In this meaning, might, may and could are very similar, and you can usually choose any of them. But do not use can for a guess about one situation: say He might be ill, not He can be ill.
You’ve been travelling all day. You must be tired.
Anna isn’t answering her phone. She might be in a meeting.
The ticket could be in your other bag. Have you looked?
That can’t be right. Ten plus ten isn’t thirty!
Ken may not know about the party. Let’s tell him.
Someone is knocking. It must be the postman.
They can’t be hungry. They’ve just eaten.
Deduction about now: be + -ing
To guess about an action happening at this moment, use modal + be + -ing.
Listen to the noise upstairs. The children must be playing.
Sam isn’t at his desk. He might be having lunch.
Deduction or obligation?
Must has two meanings. You must wear a helmet is an obligation (a rule). You must be cold is a deduction (a logical guess). The context tells you which one it is.
Common mistakes
✗ He mustn’t be at work. His car is here.
✓ He can’t be at work. His car is here.
✗ She must to be very happy.
✓ She must be very happy.
✗ He can be in the kitchen, I’m not sure.
✓ He might be in the kitchen, I’m not sure.