Study this example situation:
You are looking for Bob. Nobody is sure where he is but you get some suggestions.
You: Where’s Bob?
He may be in his office. (= perhaps he is in his office)
He might be having lunch. (= perhaps he is having lunch)
Ask Ann. She might know. (= perhaps she knows)
We use may or might to say that something is a possibility. Usually you can use may or might you can say:
* It may be true. or It might be true. (= perhaps it is true)
* She might know. or She may know.
The negative forms are may not and might not (or mightn’t):
* It might not be true. (= perhaps it isn’t true)
* I’m not sure whether I can lend you any money. I may not have enough. (= perhaps I don’t have enough)
Study the structure:
I/you/he (etc.) may/might (not) be (true/in his office etc.)
I/you/he (etc.) may/might (not) be (doing/working/having etc.)
I/you/he (etc.) may/might (not) do/know/have/want etc.
For the past we use may have (done) or might have (done):
* A: I wonder why Kay didn’t answer the phone.
B: She may have been asleep. (= perhaps she was asleep)
* A: I can’t find my bag anywhere.
B: You might have left it in the shop. (= perhaps you left it in the shop)
* A: I was surprised that Sarah wasn’t at the meeting.
B: She might not have known about it. (= perhaps she didn’t know)
* A: I wonder why Colin was in such a bad mood yesterday.
B: He may not have been feeling well. (= perhaps he wasn’t feeling well)
Study the structure:
I/you/he (etc.) may/might (not) have been (asleep/at home etc.)
I/you/he (etc.) may/might (not) have been (doing/waiting etc.)
I/you/he (etc.) may/might (not) have done/known/had/seen etc.
Sometimes could has a similar meaning to may and might:
* The phone’s ringing. It could be Tim. (= it may/might be Tim)
* You could have left your bag in the shop. (= you may/might have left it…)
But couldn’t (negative) is different from may not and might not. Compare:
* She was too far away, so she couldn’t have seen you. (= it is not possible that she saw you)
* A: I wonder why she didn’t say hello.
B: She might not have seen you. (= perhaps she didn’t see you; perhaps she did)