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

Created by , British English teacher.

Try the activities, then check your answers.

Try the activities, then check your answers.

Match your confidence to the evidence. Must expresses a strong deduction, not a proven fact. A single clue often supports might or could instead.

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.

A possible negative is different from a strong negative conclusion

One possible explanationShe may not have seen the message.

Perhaps she did not see it. Other explanations remain possible.

A conclusion from contrary evidenceThat can’t be Jo.

The speaker has strong evidence against identifying that person as Jo.

May not expresses a possibility. Can’t expresses a strong negative deduction. Mustn’t usually gives a prohibition, so it does not replace can’t in this deduction pattern.

A mystery with changing evidence

Clue 1: The path outside is wet. It might have rained, but someone could have watered it.

Clue 2: A rain gauge contains fresh water, the whole street is wet and your neighbour heard heavy rain overnight. It must have rained is now a much stronger deduction.

Keep alternatives open when the evidence is limited. Can’t expresses a strong negative deduction; it is not the same as mustn’t, which commonly expresses prohibition.

The adult woman with the approved lace-collar reference face studies a wet garden path on a dry morning. A watering can nearby supplies an alternative explanation. Her thoughtful expression conveys uncertainty; show the limited evidence without reconstructing rain as an established fact.
It might have rained. Someone could have watered the garden.Might and could leave more than one explanation possible.

See the pattern

CertaintyPresentPast
Strong yesmust be / must knowmust have been / known
Possiblemay/might/could bemay/might/could have been
Strong nocan’t be / can’t knowcan’t have been / known
Possible nomay/might not bemay/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.

might or could is a cautious choice. Wet ground is one clue, but watering or cleaning could also explain it. Use must have rained only when wider evidence supports a strong deduction.

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.

The same adult woman examines a rain gauge containing fresh water beside a broadly wet street while an adult neighbour talks to her. These multiple clues support a stronger inference about overnight rain. Depict the present evidence and conversation, not a guaranteed omniscient view of the past. No readable measurement marks.
It must have rained because the gauge has fresh water and the neighbour heard rain.Must have marks a strong deduction from several clues, rather than direct proof.

Final check

Check your understanding

Choose one answer for each question. Read the context carefully: a question may ask for a particular form, meaning or register. The explanations show why that answer fits.

0 of 10 questions answered

1. Choose the modal for the speaker’s strong deduction: “The office is dark and locked. Everyone ___ have gone home.”
2. I am not certain, but this key ___ open the back door.
3. That ___ be the correct answer; it contradicts the data.
4. Sam arrived very late. He ___ have missed the bus.
5. She speaks fluent Korean. She ___ have lived there.
6. This coat is far too small for Amir. It ___ be his.
7. Use must for a strong past deduction: “They knew the confidential details. Someone ___ told them.”
8. Which means “perhaps she did not understand”?
9. You saw Mei in Madrid yesterday? It ___ have been her: she was with me in Bogotá.
10. There are several explanations. The error ___ be a software problem.