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.

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.