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

• talk about possibility and ability
• make requests
• ask for or give permission

Structure of Can

subject + can + main verb

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

Notice that:

• Can is invariable. There is only one form of can.
• The main verb is always the bare infinitive.

Uses

Can is used to talk about ability and possibility, to ask for and give permission, and to make requests and offers.

To talk about theoretical possibility

We can use can to talk about ‘theoretical’ possibility – to say that situations and events are possible theoretically.

• Glass can be blown. (It is theoretically possible to blow glass.)
  
• Wars can break out any time. (It is theoretically possible for wars to break out any time.)
  
• Smoking can cause cancer.
  
• Noise can be quite a problem when you are living in a city.
  

Note that we do not use can to talk about future probability – to say that something will happen in future. We express this idea with may or might.

• It may rain this evening. (NOT It can rain …)
• There may be a strike next week. (NOT There can be a strike …)
• I may get a job soon.

Note that might expresses a less definite possibility than may. Could is also used in the same sense.

• It could rain this evening. (= It might rain this evening.)

To talk about logical possibility

Can is often used in questions and negatives to talk about the logical possibility that something is true.

• There is the doorbell? Who can it be?

With this meaning can is not possible in affirmative clauses. Instead, we use could, may or might.

• Where is John? He could/may/might be in the garden. (NOT He can be in the garden.)

To talk about ability

We can use can to talk about present or general ability – to say that we are capable of doing something.

• I can speak 10 languages.
  
• She can cook well.
  
• Can you knit?
  
• If you are not satisfied with this product, you can send it back.
  

Note that be able to can often be used with similar meanings.

• He is able to support her. (= He can support her.)
• They were able to catch the thief. (= They could catch the thief.)

Cannot (also can’t) shows inability.

• I can speak French, but I cannot write it.
  
• Most people cannot read traffic signals.
  
• I can't drive.
  

We do not use can to talk about future ability. Instead, we use will be able to or other words.

• Someday scientists will be able to find a cure for cancer. (NOT Someday scientists will can find a cure for cancer.)
  

To ask for or give permission

Can is sometimes used to ask for and give permission. Some people, however, think that maycan. is more correct than

• Can I use your car, John?
  
• Can we park over there?
  
• You can go out and play after you have finished your homework.
  
• You can park on either side of the road here.
  
• Can I go to the movies, mom?
  

Note that we can also use could to ask for permission. It is a more polite form of can .

• Could I speak to Mr. John, please?
  
• Could I have look at your newspaper?
  

Cannot is used to refuse permission.

• Can I go to the movies, mom? No, you can’t.

To make requests and offers

Can is used in polite requests and offers of help.

• Can you turn that music down? I am trying to work.
  
• Can you get me a cup of coffee?
  
• Can I help you with those bags?
  

Note that Could is a more polite way of making requests and offers.

• Could you help me with my homework?
  
• Could you lend me some money?