00 / Modal Verbs

Deduction (present)

Modal verbs of deduction allow speakers to express how certain they are about a present situation based on available evidence.

B1Intermediate

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.

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.

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.

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.

The phone is ringing. It ___ be my mother; she said she would call at this time.
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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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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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Answer: can't
Why: It is logically impossible or highly unlikely to know everyone after only one day.