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
Understand · You say · Check
A clear route through the lesson
Match your confidence to the evidence. Must expresses a strong deduction, not a proven fact. A single clue often supports might or could instead.
Read the explanations, answer aloud at each You say pause, then reveal the model and check one point. A model is one possible answer. Finish with the 10-question understanding check.
Keep the learner’s speaking time. At each You say pause, use the suggested follow-up and one correction focus. Finish with the 10-question understanding check.
By the end of this lesson, you can…
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 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.
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 |
Three mistakes to avoid
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.
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