、。
My father can swim, but my mother can't.
Literal
Father [topic-は] can-swim [but-が], mother [topic-は] cannot-swim [polite-ません].
A clean parallel structure contrasting two people's abilities. 泳げる is the potential form of 泳ぐ. The が softens the contrast between the two clauses. A textbook example of the potential form in a comparative context.