Can - Could

 Both Can and Could are Modal Verbs. In general Could is considered more polite (or formal) that Can.

CAN

The three main uses of Can are: 1) ability 2) possibility and 3) permission. However there are some other used of Can as we will see below.

1. To express ability

Can means to be (physically) able to do something OR to know how to do something.

• Birds can fly.
• Elephants can't fly.
• I can speak two languages.

2. To express a possibility (in general)

This refers to a theoretical possibility.
Note: we don't use can to talk about future possibilities. For this you would use may or might.

• It can get cold there at night so take a jacket.
• I know you can win the competition.
• Well, I think your car can be repaired, but it's not going to be cheap.

3. To offer to do something for others

Can is used when you offer to help someone or to do something for them.

• You don't need to walk home. I can take you there if you like.
• Can I carry your bags for you?

4. To ask for or give permission / To request something

Can is used to ask for / request permission or to give permission.
Note: Can't is used to refuse permission.

• You can use my umbrella, I don't need it right now.
• Can I sit in that chair please?
• Mary, you can play outside if you like.
• Can I ask you a question?
• You can't go to the park. It will still be wet from the rain last night.
• We can't leave the room until the assignment is completed.

5. Can't: Something that is forbidden or not allowed

• We cannot park the car next to this fire hydrant.
• You can drive as fast as you want on the German motorways.
• You can't smoke in the restaurant.
• You cannot drive a car without a license.

6. Can't: when you are sure that something isn't true or something is surprising

• It can't be dark outside! It's only 4.30pm!
• They can't have landed on the moon, I'm sure it's a hoax.

For more examples, including how to make questions, see our grammar notes about Can - Cannot. 

COULD

Could is similar to Can and often replaces Can in the past tense (though not always)

1. To express ability in the past

• I could ride a horse when I was younger but now I can't.
• She could juggle eight balls when she was only 10 years old.
• He could read when he was three years old.

2. Polite word used to ask for permission or to request something (in the present)

• Could I please use your bathroom?
• Could we move on to the next topic now please?
• Could you pass me the salt please?
• I'm busy right now. Could you call back later?

3. General permission in the past

• In high school, we couldn't leave the classroom without a pass.
• He couldn't go to the concert because his mother wouldn't let him.

4. A suggestion when asked what to do (choices and opportunities)

• He could try and fix it himself.
• We could go to the movies if you are interested.

5. Couldn't: Sure that something is untrue

Just like can't, you can use couldn't when you are sure that something isn't true or real.

• That couldn't be my cat meowing outside, it was hit by a car last week.
• He couldn't have painted that. He has no artistic ability at all.

6. Could + have + past participle: To express a possibility in the past

To express that something was possible but did not happen.

• We were lucky because it could have rained but it didn't.
• Why did you do that? You could have broken your leg.

7. Could + have + past participle: Unrealised past ability

This is used to say that someone was able to do something, but they did not try to do it. Sometimes this is a form of criticism.

• She could have married anyone she wanted to.
• Why did you sit there doing nothing? You could have helped me.

8. Could: A conditional form of Can

Would + Can = Could. Note: Would be able to can replace could in the following examples.

• If we had some oranges I could make you some fresh juice.
• I could write the letter if you told me what to include.

Can vs Could Summary Chart