Modals
May and Might: we use either of these words to say that something is possible, for example:
- I may/might be working today = It is possible that I will work today, I don't know yet
- I may/might not be working today = It is possible that I will not work today, I don't know yet
In the past:
- She may/might have worked yesterday = It is possible that she did not work yesterday, I don't know yet
- She may/might not have worked yesterday = It is possible that she did not work yesterday, I don't know yet
Note: to form the negative 'not' is placed immediately after may/might.
Could and Couldn't: could is very similar to May and Might, for example:
- You could have left your purse at home = It is possible that you left it there
- You couldn't have left your purse at home = It is not possible that you left it there
However, couldn't can also mean that something is not possible, for example:
- Sarah couldn't have got my message.
- Otherwise she would have replied. = It is not possible she got my message
- I wonder why Sarah has not replied. = She might not have got my message, perhaps she
did and perhaps she didn't.