If you use could to refer to a past ability, then the present tense of could is can.
Can is a modal verb and must be followed by an infinitive.
Example: I can swim. (present tense) ; I could swim when I was four. (past tense)
Please note that could can also be used in present tense to express possibility, permission, polite request.
Example: They could come later. Could I use your pen? Could you open the window, please?
You can read more about modal verbs and do a quiz here.