Study this example situation:
You are looking for Ben. Nobody is sure where he is, but you get some suggestions.
(= perhaps he is in his off ice)
(= perhaps he is having lunch)
(= perhaps she knows)
We use may or might to say that something is possible. You can use may or might:
- 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:
- It may not be true. (= perhaps it isn’t true)
- She might not know. (= perhaps she doesn’t know)

Note the diff erence between may be (2 words) and maybe (1 word):
- It may be true. (may + verb)
- ‘Is it true?’ ‘Maybe. I’m not sure.’ (maybe = it’s possible, perhaps)
For the past we use may have … or might have … :
- a: I wonder why Kate didn’t answer her phone. b: She may have been asleep. (= perhaps she was asleep)
- a: I can’t find my phone anywhere. b: You might have left it at work. (= perhaps you left it at work)
- a: Why wasn’t Amy at the meeting yesterday? b: She might not have known about it. (= perhaps she didn’t know)
- a: I wonder why David was in such a bad mood yesterday. b: He may not have been feeling well. (= perhaps he wasn’t feeling well)

could is similar to may and might:
- It’s a strange story, but it could be true. (= it is possible that it’s true)
- You could have left your phone at work. (= it’s possible that you left it there)
But couldn’t (negative) is diff erent from may not and might not. Compare:
- Sarah couldn’t have received my message. Otherwise she would have replied. (= it is not possible that she got my message)
- Why hasn’t Sarah replied to my message? I suppose she might not have received it. (= it’s possible that she didn’t receive it – perhaps she did, perhaps she didn’t)