Can or may? Can or could? May or might?
'Can', 'could', 'may' or 'might' are modal verbs that we use to talk about possibility, permission and requests. But it doesn't mean that they are interchangeable. Each verb has its own meaning.
If you want to read more about all Modal Verbs tap here.
Modals of deduction
These modal verbs are used to express probability and a level of certainty. In other words, we take a certain verb depending on how certain we are.
'Can', 'could', 'may' or 'might' can be used as modals of deduction. Also, we can add 'must' and 'can't' to this list.
'Can' expresses strong possibility while 'could' expresses weak possibility.
Compare these examples:
Carol can't be at work. (I'm sure she isn't at work)
Carol may/could/might be at work. (I'm not sure that she is at work)
Carol must be at work. (I'm sure she is at work)
Note that we do not use 'can' for specific probabilities and possibilities. We use it for general possibilities. For example:
It can be hot in summer in this region.
Carol could/can be at work.
Modals of permission
We can ask for permission and we can give permission. We use 'can', 'could' and 'may' in the first case. And only 'can' and 'may' in the second one.
Can I open the window? - informal
Could I open the window? - more polite
May I open the window? - very formal
You can open the window. - informal permission
You may/could open the window. - formal polite permission
Modals of request
Modal verbs 'can' and 'could' can be used to make a request. 'Could' sounds more polite than 'can'. We do not use 'may' for requests.
Can/could/may you open the window, please?
Can/could/may you repeat the question, please?
Can/could/may you give me the book, please?
Can/could/may you turn the light on, please?