B1 Intermediate Deduction and question tags
Modals of deduction — present
Lesson goal. After this lesson you can make deductions about the present with must be, might be and can't be at the right level of certainty.
When to use it
- Drawing a conclusion from evidence: The light is on, so he must be at home.
- Suggesting a possibility: He might be asleep.
- Ruling something out: She can't be a doctor — she's only ten.
Form
| Certainty | Form | Example |
|---|---|---|
| almost certainly true (about 95%) | must + be / base verb | He must be tired. |
| possible (about 50%) | might / may / could + verb | He might be at work. |
| almost certainly not (about 95%) | can't + verb | That can't be true. |
| possibly not | might not + verb | She might not know. |
Step by step
- Look at the evidence you have.
- Strong evidence, only one explanation → must.
- Weak evidence, several explanations → might / could.
- Evidence rules it out → can't, never mustn't.
must and can't in two roles
| Sentence | Role |
|---|---|
| You must wear a helmet. | obligation |
| He must be at home. | deduction |
| You mustn't smoke here. | prohibition |
| He can't be at home. | negative deduction |
Examples
The light is on, so he must be at home.
She's been working all night. She must be exhausted.
He isn't answering. He might be asleep.
That can't be Nam — he's in Ha Noi today.
You can't be hungry — you've just eaten!
She might not know about the meeting.
Common mistakes
He mustn't be at home — his car has gone. → He can't be at home — his car has gone.
mustn't means forbidden, not certainly not. Negative deduction always uses can't.
He must to be tired. → He must be tired.
No to after must.
That can be true! → That can't be true!
To say impossible you need n't. can be means it is possible.
Memory tip
Put the three on a ladder: must at the top (surely yes), might in the middle, can't at the bottom (surely no). The evidence picks the rung, not your wish.
Summary
- must be surely · might/could be possibly · can't be surely not.
- Negative deduction is can't, never mustn't.
- They are all followed by a base verb, with no to.
Practise this lesson
Do exercises with an explanation for every question, take mini tests and track your progress on GrammUp.