Can vs. could
1. Ability
I can play the guitar.
He can lift heavy weights.
She can beat anyone at chess.
Could - past tense of can
I could play the guitar when I was in school.
He could lift heavy weights until he had the accident.
She could beat anyone at chess in her prime!
2. Possibility - could
The train could be late. (present)
If you don't study, you could fail the exam. (future)
Robert could have gotten the job if he had gone to the interview. (past)
Can - general possibility
You can catch a cold if you go out in the rain.
Travelling across Europe can be quite expensive.
3. Polite expressions
(a) Making suggestions - could
We could try that new Italian restaurant tonight.
You could ask your sister to lend you some money.
(b) Making requests - can and could
Can / Could you pass me the salt (please)?
Can / Could I speak to Sandra (please)?
(c) Making offers - can and could
Can / Could I get something to drink (sir)?
Can / Could I help you with that (madam)?
(d) Ask for permission - can and could
Can / Could I borrow your pen (please?
Can / could I take the day off tomorrow?