Can or Could

The use of Can or Could depends on the context.

1. Abilities

When we talk about past abilities, we use could.
For example,
I could walk on fire.

2. Distance in a relationship

Close relationship
- Can you pass me the salt?

Distant  person
- Could you pass me the salt?

3.Conditional sentence

Probable
- If I accept the job I can buy a new car.

Improbable
- If I won lottery I could buy a new car.
"Could you pass the salt?"

"Can you pass the salt?"

The same relationship also exists between other modal pairs eg:

will-would
shall-should
may-might