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Can is an auxiliary verb, a modal auxiliary verb. We use can to:

Structure of can

The basic structure for can is:

subject + auxiliary verb
+ main verb

The main verb is always the bare infinitive (infinitive without to).

subject auxiliary verb
main verb  
+ I can play tennis.
- He cannot play tennis.
? Can you play tennis?

Notice that:

The main verb is always the bare infinitive (infinitive without to). We cannot say: I can to play tennis.

Use of can

can for possibility and ability

We use can to talk about what is possible, what we are able or free to do:

Normally, we use can for the present. But it is possible to use can when we make present decisions about future ability.

  1. Can you help me with my homework? (present)
  2. Sorry. I'm busy today. But I can help you tomorrow. (future)

can for requests and orders

We often use can in a question to ask somebody to do something. This is not a real question - we do not really want to know if the person is able to do something, we want them to do it! The use of can in this way is informal (mainly between friends and family):

can for permission

We sometimes use can to ask or give permission for something:

  1. Can I smoke in this room?
  2. You can't smoke here, but you can smoke in the garden.

(Note that we also use could, may, might for permission. The use of can for permission is informal.)