B2 grammar · teens and adults
Must, might and can't (deduction)
Guessing from evidence: She must be at work. He might be stuck in traffic. That can't be right. How sure you are, in one small word, now and in the past.
Teens · AdultsCEFR B2 · Upper-intermediate
- Age
- Ages 14+
- Level
- B2
- Skill
- Grammar
- Format
- Teacher guide
The rule
- Must: you are almost sure something is true, because of the evidence. The lights are off — they must be out.
- Can't: you are almost sure something is not true. She can't be 40 — she looks 25. For a negative deduction, use can't.
- Might, may and could: it is possible, but you are not sure. He might be in a meeting.
- After the modal comes the bare verb, be, or be + -ing: They might know. She must be working.
- For the past: modal + have + past participle. He must have forgotten. She can't have seen us. They might have missed the bus.
Form
| Use | Form |
|---|---|
| Almost sure: yes | must + verb · must have + past participle |
| Possible | might / may / could + verb · might have + past participle |
| Almost sure: no | can't / couldn't + verb · can't have + past participle |
| In progress | must / might be + verb-ing |
Example sentences
Tap a sentence to hear it. The translation follows each one.
Quick check: choose the right form
Five sentences from this page, each with one gap. Choose the form that fits; when it is right, you hear the whole sentence.
You must blank tired after such a long flight.
The office is dark. They must blank gone home.
She can't have blank my message yet.
I might have blank my keys at the gym.
They must be blank a party next door.
Common mistakes
What learners actually say, why, and which first language makes it typical.
Typical for Russian and Spanish speakers
She must to be at work.She must be at work.
Modals take the bare verb. Russian «должна быть» has an infinitive, and learners translate «быть» as to be; Spanish «debe de estar» has a de between the verbs, which turns into to.
He must have went home.He must have gone home.
After have, use the past participle (gone), not the past simple (went).
Typical for Russian speakers
He mustn't be at home — the lights are off.He can't be at home — the lights are off.
In British English mustn't means “not allowed”; for “I'm sure it isn't true”, use can't. Russian «не должно быть» invites mustn't. (American English sometimes says must not here, but can't is the safe choice everywhere.)
Typical for Spanish speakers
It can be the courier — someone is at the door.It could be the courier — someone is at the door.
Spanish «puede ser el mensajero» leads to can, but can talks about what is generally possible (It can be cold in May). For a guess about one situation, use could, might or may.
How to teach it
- Mystery bag or mystery photo: show part of a picture or a few things from someone's bag. Learners deduce: She must be a doctor. He can't be a student — look at that watch.
- Draw a certainty line on the board: can't (0%) — might / could (50%) — must (100%). Learners place their guess on the line before they say it.
- Past deduction with “what happened here?” photos: The window is broken — someone must have climbed in. Teens love crime-scene versions.
Questions teachers ask
Why not mustn't for a negative deduction?
Mustn't normally means “not allowed”. For “I'm sure it isn't”, British English uses can't (or couldn't); American English sometimes says must not, but can't is understood everywhere and is what exams expect.
Might, may or could: is there a difference?
For present possibility they are almost the same; may is slightly more formal. In the negative they split: might not and may not mean “perhaps not”, but couldn't means “impossible”.
Related resources
Presentation
Modals of Deduction Presentation (Free PPT)
Modals of Deduction presentation (B2) for teens and adults: the rule, a form table, 10 examples and practice with answers. Free PPTX and PDF.
Worksheet
Must, might and can't (deduction) — worksheet
Put the verb in the right form, build the sentence, choose the correct one — answers at the bottom.
Lesson Plan
Used to and would
Past habits and old states: I used to live in Lisbon. We would stay up all night. When both work, when only used to does, and the be used to trap.
Lesson Plan
Relative clauses
Who, which, that, where, whose: adding information about a noun. Defining and non-defining clauses, when to drop the pronoun, and why commas matter.