Can and could are both used to talk about ability, to ask for and give permission and to make requests and offers. As you will see be able to and was able to can be substituted for ability and permission.

  • Requests

Could is more polite than Can

Could I have a glass of water? (Can I have…? is stronger)

  • Permission

As Can is stronger than Could permission is given with Can.

Could I have a glass of water? Yes, you can. (not Yes, you could)

  • Past

Could is used to talk about ‘general ability’ in the past:

When I was a teenager I could run 100 metres in under 12 seconds.

For ‘specific ability’ on one occasion we use: be able to

I was able to to run 100 metres in 16 seconds yesterday.

Could is used to talk about ‘general permission’ in the past:

When I was a teenager I could the family car.

For ‘specific permission’ on one occasion we use: be allowed to

I was allowed to stay up late last Saturday night.

  • Future

Can is used to talk about the future if we talk about current abilities etc.,

She can win the race next Saturday. (she is a good runner now)

In other cases we use be able to for ability:

I will be able to drive a car when I finish the course. (I cannot drive now.)

and be allowed to for permission:

I will be allowed to drive a car when I finish the course. (I will have a driving licence.)

1. can

Use Examples Examples
ability to do sth. in the present I can speak English. I am able to  speak English.
permission to do sth. in the present Can I go to the cinema? Am I allowed to go to the cinema?
request Can you help me, please?
offer I can help you with that.
suggestion Can we visit London soon?
possibility It can get very cold in winter.

2. could

Use Examples Example
ability to do sth. in the past I could speak Spanish when I was four. I was able to buy a nice coat yesterday
permission to do sth. in the past When I was young I could go to the cinema by nyself. I was allowed to go to the cinema last night
polite question * Could I go to the cinema, please?
polite request * Could you help me, please?
polite offer * I could help you with that.
polite suggestion * Could we visit London soon?
possibility * It could get very cold in winter.

There is no contracted form of can in affirmative sentences.

Affirmative sentences Negative sentences
long form contracted form long form contracted form
I can sing. I cannot sing. I can‘t sing.
You can sing. You cannot sing. You can‘t sing.
He can sing. He cannot sing. He can‘t sing.
She can sing. She cannot sing. She can‘t sing.
It can sing. It cannot sing. It can‘t sing.
We can sing. We cannot sing. We can‘t sing.
You can sing. You cannot sing. You can‘t sing.
They can sing. They cannot sing. They can‘t sing.