Modal verbs features: Modal verbs have more than one meaning: I can swim ability

Couldn’t have (done) is possible instead of can’t have (done):

She couldn’t have seen me, because she didn’t say “hello”.

• 
  We use may/might as well to say that we should do something but only because there is no reason not to do it and because there is nothing better to do. It means “Why not to do it? There is nothing better to do.”
  

You’ll have to wait an hour for the next bus, so you might as well walk.

UNIT 11. REQUESTS ,PERMISSION, OFFERS AND INVITATIONS

• 
  We often use can or could when we ask someone to do something:
  

Can you wait a moment, please? Excuse me. Could you tell me how to get to the station?

• 
  To ask for something you can say Can I have…?/ Could I have…?/ May I have…?
  

(in a shop) Can I have these postcards, please?
(at table) Could I have the salt, please?

• 
  We often use can, could or may to ask permission to do something:
  

(on the phone) Hello, can I speak to Tom, please?
Could I use your telephone?
May I come in?

To give permission we use can or may (but not could):

You can smoke if you like.

Can is used to talk about what is already allowed or not allowed. You can use be allowed to instead of can in this meaning:
We can have mobile phones in my office, but we can’t use them during the meeting.
We are allowed to have mobile phones in my office, but we aren’t allowed to use them during the meeting.

May is especially used when we ask permission from someone to do something:
Mummy, may I go out with Ted, please?

• 
  We sometimes use can when we offer to do things:
  

Can I get you a cup of coffee?

• 
  For offering and inviting we use Would you like…? (not do you like):
  

Would you like a cup of coffee?