Can expresses ability. Cannot (can’t) shows inability.
- She can speak ten languages.
- I can’t cook.
- Can you speak Spanish?
Sometimes can is used in the sense of may to give permission.
- You can go. OR You may go.
- You can take one of these shirts. OR You may take one of these shirts.
Now-a-days can is also increasingly used to ask permission.
- Can I go? OR May I go?
Could is the past tense of can. It is used to talk about ability that existed in the past.
- In my younger days I could run four miles at a stretch.
- Till last year I could read without glasses.
Note that could doesn’t always refer to past time. It refers to past time only when the context makes the time clear.
Could is the past tense of can in indirect speech.
- He said, ‘ I can lift this box.’
- He said that he could lift that box.
- She said, ‘I can’t see anything.’
- She said that she couldn’t see anything.
Possibility or uncertainty
Could may express possibility or uncertainty.
- You could do it, if you tried hard.
- If my brother were here, we could have solved this problem together.
Could is also used to ask polite questions.
- Could you, please, take me to the Manager?
- Could I have a look at your papers?
Can and could are followed by infinitives without to.
- I can knit. (NOT I can to knit.)
- She could understand nothing. (NOT She could to understand nothing.)
Questions and negatives are made without do.
- Can he speak English? (NOT Does he can speak English?)
- He can’t speak English. (NOT He can doesn’t speak English.)
There is no -s in the third person singular.
- She can sing. (NOT She can sings.)