B1 Grammar: Modals of Deduction

B1 Grammar: Modals of Deduction
ESL Structure Curriculum

ESL Grammar Course

B1: Intermediate

A scale showing different levels of certainty, from impossible to certain

Modals of Deduction

We use modals of deduction to make logical guesses about a present situation based on evidence. The modal we choose shows how certain we are about our guess.

Levels of Certainty:

  • `must be`: Almost 100% certain (positive deduction).
  • `might be` / `could be`: Possible, but not certain (around 50%).
  • `can’t be`: Almost 100% certain something is impossible (negative deduction).

⚠️ Attention: Common Challenges

1. `Can’t be` vs. `Must not be`: For negative deduction (when you are sure something is impossible), always use `can’t be`. `Must not` is used for prohibition, not deduction.

  • “He has a big coat and gloves. He can’t be hot.” (Deduction)
  • “You must not smoke here.” (Prohibition/Rule)

2. Structure: The structure is always the same and simple.

  • `must / might / could / can’t + be + adjective/noun`
  • `must / might / could / can’t + be + verb-ing`

Practice Quiz

1. The lights are on in their house. They ____ be at home.

2. He just ate a huge lunch. He ____ be hungry already.

3. I’m not sure where Sarah is. She ____ be at the library, or maybe she’s at the gym.

4. You haven’t slept for 24 hours. You ____ be exhausted.

5. That’s a strange noise. It ____ be the wind.

6. She drives a very expensive car and wears designer clothes. She ____ have a lot of money.

7. That ____ be the right answer. It doesn’t make any sense.

8. He’s not answering his phone. He ____ be sleeping.

9. This isn’t my coat. It’s too small. It ____ belong to you.

10. He lives in a tiny apartment and has no job. He ____ be rich.