Don't mean to contradict the person before me, but that's not entirely true! While could and couldn't are past forms of can, they can be used in present tense as well for a different purpose. For example:
"He could do it if he tried."
"Could you get me some water?"
"I could sleep for days after that run."
In those cases it's used to express a possibility or to be polite, all in present tense.