Deduction (present)
B1 IntermediateModal verbs of deduction allow speakers to express how certain they are about a present situation based on available evidence.
Degrees of Certainty
We use modal verbs to make guesses or logical conclusions about the present. The choice of modal depends on the strength of the evidence:
- Must: Used when you are almost 100% sure something is true because the evidence is very strong.
- Might / Could / May: Used when something is possible, but you are not certain.
- Can't: Used when you are almost 100% sure something is impossible.
Structure
Subject + must/might/could/can't + verb (base form)
Modal verbs of deduction are followed by the base form of the verb (infinitive without 'to'). They do not change form for different subjects.
Must vs. Can't
Note that the opposite of must for deduction is can't, not mustn't.
| Modal | Meaning | Example |
|---|---|---|
| Must | Logical positive certainty | They have three cars; they must be rich. |
| Can't | Logical negative certainty | He just ate a huge meal; he can't be hungry. |
Examples in Context
Common Mistakes
It mustn't be true because he never lies.
It can't be true because he never lies.
Use 'can't' for negative deduction. 'Mustn't' is used for prohibition (rules).
She coulds be at work now.
She could be at work now.
Modal verbs never take an -s in the third person singular.
Practice
The phone is ringing. It ___ be my mother; she said she would call at this time.
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must
Answer:must
Why:The speaker is very certain because there is specific evidence (the mother's promise).
Take an umbrella. It ___ rain later, though the sky is clear now.
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might
Answer:might
Why:Rain is a possibility, but the clear sky means the speaker is not certain.
He only started the job yesterday, so he ___ know everyone's name yet.
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can't
Answer:can't
Why:It is logically impossible or highly unlikely to know everyone after only one day.