the past tense of can1, used:
to express possibility in the past
He realized he could go
to express a condition
He said he would go if he could
to express a possibility
You could be right
as a polite form in the present
Could you do this for me?
to express expectation or obligation
You could at least apologize
[Middle English coude from Old English cuthe, past tense of cunnan can1]