When can we use them?

We use the modal verbs must, may, might, could or can’t to speculate about events

in the past.

LESS CERTAIN: may, might and could

MORE CERTAIN: must

Certain that the statement is false: can’t, couldn’t

We use must to make deductions when we are almost sure or more certain.

For past deductions we use must + have + past participle:

He’d been working for 10 hours when I saw him. He must have been

very tired.

We use can’t to make negative deductions from evidence.

.

For past we use can’t + have + past participle:

He can’t have failed the exam. He’s a very good student!

We use may/ might / could to make speculations (i.e. when there is no evidence).

We use may / might / could + have + past participle for past:

He may/might/ could have phoned while you were out. The answer phone was switched off.

Deductionmust have and can't have + past participle

The plane ________ ________ run out of fuel.

It _________ ________ blown up in mid-air.

Speculationcould/may/might have + past participle

What could ________ happened to them?

They could _____ _____ up on a desert island.

Earhart and Noonan may _____ _____ US spies.Aliens _______ _______ abducted them.

We use ________ ________ when we are sure something happened.

We use ________ ________ when we are sure something didn't happen.

We use _______ _______ ______ ______ to talk about what possibly happened.