The Grammaring Guide to English Grammar with Exercises

Modals to express deduction: MUST, CAN'T, COULDN'T

Present

Must, can't and couldn't + infinitive are used to express deductions about present events:

That child is really talented. His parents

must be

proud of him.

It's only 10 o'clock. He

can't/couldn't be

at home.

Past

Must, can't and couldn't + perfect infinitive are used to express deductions about past events:

That dress looks expensive. It

must have cost

a fortune.

It

can't/couldn't have been

Carla you saw this morning. She is away on holiday.

Related topics:

Modals to express possibility: MAY, MIGHT, CAN, COULD