Modal Verbs of Deduction

• CEFR B2
• Grammar
• Printable
• Answer key

Modal Verbs of Deduction is a printable grammar worksheet for English language learners at CEFR level B2. It covers must might can't, with 5 exercises and an answer key.

We use modal verbs to make deductions — logical conclusions based on evidence. 'Must' expresses near certainty that something is true. 'Can't' (or 'couldn't') expresses near certainty that something is NOT true. 'Might' or 'could' expresses a possibility — we are not sure.

For present deductions: must / can't / might + base verb ("She must be tired."). For past deductions: must / can't / might + have + past participle ("She must have been tired yesterday.").

The strength of certainty: must (almost certain positive) → might / could (possible) → can't (almost certain negative). None of these express 100% certainty — they are all conclusions from evidence.

Examples

• She isn't answering her phone — she must be busy.
• He's been awake since 4am. He must be exhausted.
• That can't be right — the answer was 42, not 24.
• They left three hours ago — they might have arrived by now.
• She studied for months. She can't have failed.
• Nobody's home. They must have gone away for the weekend.
• I'm not sure where he is — he could be at the gym.
• The window is open and the keys are missing. Someone must have been here.

What's in this worksheet

1. Choose the correct modal
2. Correct the mistake
3. Make deductions
4. Write the deduction
5. Gap fill

Common mistakes

• She must have tired after the marathon.  →  She must have been tired after the marathon. Past deduction: must have + past participle. 'Tired' is an adjective, so the past participle of 'be' ('been') is required.
• He can't have not been at the office.  →  He can't have been at the office. Double negative: 'can't' is already negative — don't add 'not have'.
• They might arrived late.  →  They might have arrived late. Past deduction = might HAVE + past participle. Don't omit 'have'.
• She must be left her bag on the train.  →  She must have left her bag on the train. Past deduction needs 'have' after the modal. 'Must be left' mixes present and past forms.

Speaking prompts

• You come home to find your front door open, a half-eaten sandwich on the table, and a book left face-down on the sofa. What do you think happened?
• Your friend hasn't replied to your messages in three days. What could have happened?
• A very expensive painting has disappeared from a locked gallery. What are some possible explanations?
• You find an old, unopened letter addressed to you in a drawer. What might it contain?