When should you use 'can' and 'could'?

The verb 'could' is the past tense of the verb 'can'.

The verbs 'can' and 'could' are most often auxiliary (helping) verbs used to express possibility or ability to do something.

Examples:

We can bring some food for a picnic. (auxiliary verb)

Yes, we can. (verb)

I could dance all night when I was twenty. (auxiliary verb)

I never could. (verb)