Deductions

1. Certainty – present

To express that we are very sure about something in the present, we can use must .

You’ve worked all day. You must be very tired!

You must be the new boss. Nice to meet you.

To express that we are sure something is not possible in the present, we use can’t and couldn’t.

You can’t be tired! You’ve been sleeping all day.

It couldn’t be true! He always lies.

Note: mustn’t (obligation, uncertainty) is not the opposite of must (prohibition)!

2. Certainty – past

To express that we are very sure about something in the past, we can use must have.

Jo didn’t answer the phone. He must have fallen asleep.

Well done for passing the test. You must have studied a lot.

To express that we are sure something is not possible in the past, we use can’t have/couldn’t have.

He can’t have lent you the money. He’s mean!

He couldn’t have stolen the painting. He has an alibi.

3. Possibility – present

To express uncertainty or possibility in the present, we can use may, might or could.

‘Where’s Jane?’ ‘I don’t know. She may be in kitchen. Or she could be in bathroom.’ (The speaker is not sure.)

‘Don’t buy him that tie. He may not/might not like it.

Note: We don’t use couldn’t to express possibility.

4. Possibility – past

To express possibility in the past, we can use may have, might have or could have.

‘Why is that child crying?’ ‘He may have lost his toy.’

‘Where are the diamonds?’ ‘They could have been stolen!’

5. Probability – present

To express probability or expectation, we can use will, should (ought to) or to be bound to.

She will arrive soon. (I expect her to arrive soon because she usually arrives at this time.)

She should be in Paris now. (I expect that the plane has landed.)

He’s bound to be promoted. He’s the best. (I expect that he will be promoted.)

6. Probability – past

To express probability in the past, we can use should have or shouldn’t have.

He should have arrived in New York by now. (I expect that he has arrived.)

He shouldn’t have sold his car. He loves it. (I expect that he didn’t sell his car.