Modals of speculation about the past

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.