A modal auxiliary verb is used to modify the mood of a verb.

Can:
“Can” is used to express ability, willingness, permission,
or possibility.
The negative of can is “cannot” or the contraction “can’t”.

Example:
I can cook dinner.
In this example “Can” expresses the ability to cook dinner.

Could:
“Could” has at least three functions.

First, it can replace “can” and give the phrase a more conditional tone.

Second, it can function as the past tense of “can”.

Third, it can function in the same way as “might” or “may”, suggesting that something is a possibility.

The negative of could is “could not” or the contraction “couldn’t”.

Example:
You could have been an astronaut if you wanted to.

In this example “Could” expresses the conditional ability to be an astronaut in the past.