B1 · Modal Deduction — Lesson

Modal deduction: present

We use modals to make deductions — logical conclusions based on evidence. Three main degrees of certainty.

must be

almost certain it IS true: She must be tired — she's been working all day.

might / could be

possible — maybe 50%: He might be at home.

can't be

almost certain it IS NOT true: That can't be right — I just checked.

The certainty scale

100% certain ✓

She is tired. (fact)

~95% certain

She must be tired. (deduction)

~50% possible

She might/could be tired.

~95% impossible

She can't be tired. (deduction)

100% not ✗

She isn't tired. (fact)

For deduction, must means "I'm almost sure it's true" — not obligation (that's a different use of must).

Modal deduction: past

Modal + have + past participle

modal + have + past participle → She must have been tired. ✓

must have + PP

She must have left already — the door's locked.

might have + PP

He might have forgotten.

can't have + PP

She can't have done it — she wasn't there.

could have + PP

He could have taken a different route.

must vs can't — opposites

must be / must have

He knows everything — he must have studied a lot.

She won — she must be very talented.

can't be / can't have

He arrived in 5 minutes — he can't have taken the motorway.

That can't be her — she's in Tokyo.

Common mistakes

She must be studied hard.

→

She must have studied hard.

past deduction → must have + PP

He can't study — he was ill. (deduction)

→

He can't have studied — he was ill.

past deduction → can't have + PP

She mustn't be home. (she isn't there)

→

She can't be home.

negative deduction → can't (not mustn't)

Recap

must be

almost certain it IS true

She must be exhausted.

can't be

almost certain it IS NOT true

That can't be right.

might/could be

possible

He might be at work.

Past deduction

modal + have + PP

must have left · can't have seen

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