Possibility or certainty | Johnny Grammar | Learn English | British Council

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We use modal verbs (must, may, could, might, can't) to show how certain we are about something.

We use 'must' when we are almost certain about something - 'must + infinitive without to':

That must be Jack. He always rings at this time.

The past of 'must' is 'must have + past participle':

He must have finished work by now. He must be on his way home.


'May' is used to say that something is a possibility - 'may + infinitive without to':

I'm going to fill the bath. Jack may want a bath when he arrives. He sometimes does.

The past of 'may' is 'may have + past participle':

He may have had a shower before he left work.


'Could' is like 'may':

I suppose he could have spent the day in the office, so he may not want a bath.


'Might' is used to say something is possible but we think it's unlikely - 'might + infinitive without to':

Of course, he might stop at the pub for a drink on his way home. He sometimes does, but he didn't say so on the phone.

The past of 'might' is 'might have done / been':

He's very late. He might have had an accident, although he's a very careful driver.


We use 'can't' to say we think something is impossible and we have evidence for thinking this. 'Can't' is the negative of 'must' - 'can't + infinitive without to':

He can't be at the cinema. He never goes to the cinema without taking me.

The past of 'can't' is 'can't have + past participle':

He can't have gone with someone else. He wouldn't dare!