B2 grammar lesson

Modal Verbs of Deduction

Good speakers separate facts from conclusions. Modals of deduction show how certain you are when you interpret evidence in the present or reconstruct the past.

About 20 minutes · Guided practice · 10-question mastery test

By the end of this lesson, you can…

UnderstandExpress strong and weak conclusions without sounding absolute.
ChooseBuild present and past deductions accurately.
UseAvoid confusing negative possibility with negative certainty.

The essential rules

Strong positive conclusion

Use must when the evidence makes a conclusion very likely. It is a deduction, not an obligation.

The lights are on. Someone must be home.

Possibility

Use may, might or could when several explanations remain possible. Use may not/might not for negative possibility.

She might be in a meeting. She may not have seen the message.

Strong negative conclusion

Use can’t when evidence makes something nearly impossible. For the past, use modal + have + past participle.

That can’t be Jo. · They must have left early.

See the pattern

Certainty Present Past
Strong yes must be / must know must have been / known
Possible may/might/could be may/might/could have been
Strong no can’t be / can’t know can’t have been / known
Possible no may/might not be may/might not have been
Teacher tip: Start with the evidence, then calibrate the modal. Must is a powerful conclusion, so reserve it for evidence that rules out most alternatives.

Three mistakes to avoid

✕ He mustn’t be at work. (deduction)
✓ He can’t be at work.

Mustn’t usually means prohibition.

✕ She might has forgotten.
✓ She might have forgotten.

Use modal + have + past participle.

✕ They can have missed it. (possibility)
✓ They could have missed it.

Use could, not affirmative can, for a specific possibility.

Guided practice

Try each item before opening the answer.

1. The ground is wet, but the sky is clear now. It ___ have rained overnight.

must — the wet ground is strong evidence.

2. Alex is not answering. He ___ be driving, but I am not sure.

might/may/could — this is one possible explanation.

3. Nora is in Tokyo today, so that woman ___ be Nora.

can’t — the evidence makes it impossible.

4. Perhaps they did not receive the invitation.

They might not have received the invitation.

Final check

Mastery test

Target: 8/10

1. The office is dark and locked. Everyone ___ have gone home.

Why: The evidence strongly supports this past conclusion.

2. I am not certain, but this key ___ open the back door.

Why: Might expresses a real but uncertain possibility.

3. That ___ be the correct answer; it contradicts the data.

Why: Can’t expresses a strong negative deduction.

4. Sam arrived very late. He ___ have missed the bus.

Why: Might have + past participle expresses a past possibility.

5. She speaks fluent Korean. She ___ have lived there.

Why: May have lived is a plausible past explanation.

6. This coat is far too small for Amir. It ___ be his.

Why: The size provides strong evidence against the conclusion.

7. They knew the confidential details. Someone ___ told them.

Why: Must have told expresses a strong conclusion about a past action.

8. Which means “perhaps she did not understand”?

Why: Might not have + participle expresses uncertain negative past possibility.

9. You saw Mei in Madrid yesterday? It ___ have been her—she was with me in Bogotá.

Why: Can’t have been expresses a past impossibility.

10. There are several explanations. The error ___ be a software problem.

Why: Could marks one possibility among several.