5.13 Modals to express possibility: MAY, MIGHT, COULD

Present and future

May and might + infinitive are used to express present or future possibility. May expresses a greater degree of certainty:

You should ask him. He may/might know Susan's telephone number. (Perhaps he knows her number.)
I may/might see you later. (Perhaps I will see you later.)
You should introduce yourself; he may/might not remember you. (Perhaps she doesn't/won't remember you.)

May and might are usually not used to introduce a question. Instead, we can use Do you think?:

Do you think he may/might know Susan's telephone number?

Could can be used instead of may and might with the verb be:

You could be right.
They could still be waiting for us.

The negative form couldn't is often used with comparative adjectives:

The food is delicious, and the staff couldn't be more polite. (they are very polite)

Except for this use, couldn't expresses negative deduction, not possibility:

It's only 10 o'clock. He couldn't be at home. (He is usually at work at this time of the day.)

Past

May, might and could + perfect infinitive express uncertainty with reference to past actions:

We haven't heard from him for ten years. He may/might/could have died. (Perhaps he has died, but we don't know.)

But when we want to say that something was possible but did not happen, we use might or could:

He was very careless when crossing the road. He might/could have died. (He didn't die.)
I could have caught the bus if I had hurried. (I didn't hurry, so I didn't catch the bus.)

Couldn't + perfect infinitive is often used with comparative adjectives:

It was a great year, and I couldn't have been happier. (I was very happy)

May/might not + perfect infinitive is used for uncertainty, but could not + perfect infinitive (except for the case above) expresses deduction:

I had better call Anne. She may/might not have read my e-mail. (uncertainty)
It couldn't have been John you saw this morning. He is away on holiday. (deduction)