Modals of Deduction

Reference

Common mistakes

1. He mustn't be hungry — he just ate. → He can't be hungry — he just ate.
   negative deduction → can't
2. She must to be at work. → She must be at work.
   no to after a modal
3. It can be Tom at the door. → It could / might be Tom at the door.
   possibility about now → could / might
4. They must watch a film — it's noisy. → They must be watching a film.
   happening now → be + -ing

Modals of Deduction — Exercises

Choose the best modal

Circle the correct answer.

1. She's been running for an hour. She _____ be exhausted. (must / can't / might not)
2. That _____ be the right answer — it's far too big. (can't / mustn't / must)
3. I'm not sure where Leo is. He _____ be in the library. (might / must / can't)
4. You _____ be hungry — you haven't eaten all day! (must / can't / could not)
5. The phone is engaged. She _____ talking to someone. (must be / must / must to be)
6. Take an umbrella. It _____ rain later. (could / can't / must)
7. He _____ be at home — I can see him at the café. (can't / mustn't / might)
8. She _____ know about the party. Don't mention it. (might not / mustn't / can't to)

Complete the deduction

Write must, might, could or can't (with be if needed).

1. The baby is crying. She   hungry. (almost sure)
2. This   Maria's coat — it's much too small for her. (impossible)
3. Where's my phone? It   in the car. (possible)
4. They aren't answering. They   sleeping. (almost sure)
5. You   serious! That's a terrible idea. (impossible)
6. I don't know why he's late. He   stuck in traffic. (possible)

Modals of Deduction — Answer Key

Choose the best modal

1. must — Almost sure, yes → must.
2. can't — Almost sure, no → can't.
3. might — Possible → might.
4. must — Logical conclusion → must.
5. must be — Happening now → must be + -ing.
6. could — A possibility → could.
7. can't — Impossible → can't.
8. might not — Maybe she doesn't → might not.

Complete the deduction

1. must be
2. can't be
3. might be
4. must be
5. can't be
6. could be