It is a modal verb, and modal verbs do not have past participles. They express ability, possibility, permission, or obligation, and their meaning is usually conveyed by using auxiliary verbs like "have" and "be".
For example:
* I can swim. (Present)
* I could swim. (Past)
* I have been able to swim. (Past perfect)
As you can see, "can" itself doesn't change to indicate past tense. Instead, we use "could" for the past tense and "have been able to" for the past perfect.