fish can swim為什麼不用fishes

fish的單複數相同。一條魚：a fish 兩條魚：two fish。fishes表示多種魚，指種類。例如：

There are many fishes in the river. 河裏面有多種魚。

fish它既可以做可數名詞也可以做不可數名詞

做可數名詞時反覆同形，做不可數名詞時是魚肉的意思

例句：He likes eating some vegetables and some fish for dinner.