Requests and offers
When making a request for something, the norm is to use can or could:
- Can I have two of those balloons please?
- Could I have two of those balloons please?
The latter is more polite.
Although can and could are perfectly acceptable, may is also used in such cases, as it’s regarded as more polite and formal:
- May I have two of those balloons please?
Might is used to make requests in very formal situations and to make the request sound more like a polite suggestion:
- Might I ask you to go through the report again?
Can is the most preferred way of making an offer; could makes the offer sound more tentative; may is more formal and polite:
- Can I get you something to drink?
- Could I be of any help?
- May I get you something to drink?
Asking for or giving/refusing permission
The most common way of asking for permission is to use can, or if you want to sound more polite you can use could:
- Can I borrow your dress?
- Could I borrow your dress?
Some argue that can and could should be used only in context of ability and possibility, so it is advisable to use may in more formal writing and speaking (might is regarded as very formal and hence rarely used):
- May I borrow your dress? [formal, polite]
- Might I borrow your dress? [rare, polite, very formal]
When giving or refusing permission, only can (or can’t) and may (or may not) are acceptable:
- Yes, you can borrow my dress.
- Yes, you may borrow my dress. [more polite/formal]
- No, you can’t borrow my dress.
- No, you may not borrow my dress. [more polite/formal]