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? or be likely to/that:
Do you think he may/might know Susan’s telephone number?
Are you likely to get here before 8?
Is it likely that you will get here before 8?
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.)
Can may express general possibility:
Winters in Minnesota can be really cold.
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)