"con cá" = the fish (the subject is definite) Tôi thích con cá = I like the fish Bạn đã thấy con cá chưa? = Have you seen the fish?
"cá" = fish (the subject is indefinite) Tôi thích ăn cá = I like to have/eat fish/fishes
Con cá biết bơi = The fish knows how to swim Cá biết bơi = Fish/fishes know/knows how to swim
Please notice that in Vietnamese, in most cases animals comes with the classifier "con" in the sentence. Example: con người (the human), con gà (the chicken)