Must, Might, Can't for Guesses — Modals of Deduction Exercises
Practice modals of deduction about the present: they must be out, he can't be hungry, she might be driving, it should arrive today.
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The lights are off and nobody's answering the door. They ___ be out.
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must
The evidence (no lights, no answer) makes you sure → 'must' = I'm almost certain it's true. 'Mustn't' means something is not allowed — it's never used for guesses.
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He's just had a huge lunch. He ___ be hungry already!
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can't
You're sure it's NOT true → 'can't'. The opposite of 'must' in guesses is 'can't', not 'mustn't'.
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Anna isn't answering her phone. She ___ be driving — it's Sunday, and she often visits her mum.
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might
A possible explanation, not a certain one → 'might be driving'. 'Can't' would mean you're sure she isn't, but the context suggests she could be.
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You've been travelling all day. You ___ be exhausted.
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must
A logical conclusion from the facts (a whole day of travel) → 'must be'. 'Mustn't' is a prohibition, not a guess.
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Listen to that music! The neighbours ___ having a party.
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must be
A guess about something happening right now → 'must be' + -ing. 'Must have been' would be about the past.
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The parcel was sent on Monday, so it ___ arrive today or tomorrow.
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should
A logical expectation — what should happen if everything goes normally → 'should'. 'Must arrive' would sound like a rule, not an expectation.
Modals of Deduction: Must, Might and Can't About the Present
Modal verbs show how sure a speaker is that something is true. Must expresses a confident conclusion based on evidence: The lights are off and nobody's answering — they must be out. Can't is its opposite, a confident conclusion that something is not true: He's just had a huge lunch — he can't be hungry. Might, may and could express possibility without certainty: She might be driving. Should expresses what we expect if everything goes normally: The parcel was sent on Monday, so it should arrive today. For an action in progress, use the modal with be + -ing: The neighbours must be having a party. The most frequent error is using mustn't for a negative guess. Mustn't expresses prohibition — You mustn't park here — and is never used to say that something is probably not true.
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