Modals of Deduction

Reference

Common mistakes

1. She must be studied hard. → She must have studied hard.
   past deduction → must have + PP
2. He can't study — he was ill. (deduction) → He can't have studied — he was ill.
   past deduction → can't have + PP
3. She mustn't be home. (she isn't there) → She can't be home.
   negative deduction → can't (not mustn't)

Modals of Deduction — Exercises

Choose the right modal

Circle the correct answer.

1. She's won every race this year. She _____ be very fit. (must / can't / might)
2. That _____ be Tom at the door — he's in Australia! (can't / must / might)
3. I'm not sure where Ana is. She _____ be at the gym. (might / must / can't)
4. The streets are wet. It _____ have rained in the night. (must / can't / should)
5. He _____ have seen the film — he didn't know the ending. (can't / must / might)
6. The app has a million downloads. It _____ be really good. (must / can't / mustn't)
7. Where's my phone? I _____ have left it on the bus. (might / can't / must not)
8. She _____ be at home. I can see her in the park. (can't / mustn't / might)

Complete the deduction

Use must, might, could or can't (+ have + past participle for the past).

1. You've been walking all day. You   be exhausted.
2. The lights are off. They   have gone to bed.
3. That   be right. I've just checked it.
4. I don't know where Leo is. He   be in the library.
5. She didn't come. She   have forgotten.
6. He   have eaten all the pizza — he was only here for a minute!

Modals of Deduction — Answer Key

Choose the right modal

1. must — The evidence makes you almost sure → must.
2. can't — Almost sure it is impossible → can't.
3. might — A possibility, not sure → might.
4. must — Almost sure about the past → must have + past participle.
5. can't — Sure it did not happen → can't have seen.
6. must — Strong evidence → must.
7. might — One possibility → might have left.
8. can't — Impossible → can't. Mustn't means "not allowed".

Complete the deduction

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