Modals of deduction: must, might & can’t
We use modals of deduction to say how sure we are about something — in the present and in the past. Here’s how they work, the two mistakes almost every learner makes, and a quick practice to test yourself.
Present deduction
Use modal + base verb when you draw a conclusion about now.
✓“The lights are off — she must be asleep.” (I’m sure)
✓“He can’t be hungry, he just ate.” (I’m sure it’s NOT true)
✓“They might / may / could be stuck in traffic.” (it’s possible)
Past deduction
Use modal + have + past participle when you draw a conclusion about the past.
✓“The ground is wet — it must have rained last night.” (I’m sure)
✓“You can’t have seen her — she’s overseas.” (I’m sure it’s NOT true)
✓“He’s late; he might / may / could have missed the bus.” (it’s possible)
The two mistakes everyone makes
✗“He mustn’t be home.” → ✓ “He can’t be home.”
mustn’t = prohibition (not allowed), not negative deduction.
✗“She must of left.” → ✓ “She must have left.”
It sounds like “must of”, but it’s always have (must’ve).
Quick practice
Choose the best modal of deduction, then check your answers.
- “Her car’s gone — she ______ already left.” (you’re sure)
- “They ______ be twins — they look identical!” (you’re sure)
- “I ______ have left my keys at the café; I’m not certain.” (it’s possible)
- “You ______ have finished the whole book already — it’s 900 pages!” (you’re sure it’s not true)
Show answers
Reading a rule is one thing; using it naturally is another. In a Lexis English General English class you’ll practise structures like this every day, with a teacher correcting you in real time — and the beach a short walk away.
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