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)

Your turn

Answer out loud — full sentences

Make deductions with must, might, could or can't.

Look at a photo on your phone. Where must / might it be?

Your friend is not answering the phone. Why? (She might be…)

The kitchen is a mess and the cake has gone. What must have happened?

Describe a person in the room: what can you guess about them?

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

Practice now →

Modal Deduction