cow作为名词，解释为母牛；奶牛；菜牛；肉牛的时候

单数 cow's milk 牛奶

The cowboy splashed his way across the shallow stream with hiscow.

牧童牵着牛淌过浅溪。

复数加S a herd of dairy cows (= cows kept for their milk) 一群奶牛

The milk man selected the spotted cows,from among a herd of two hundred.

牛奶商从一群200头牛中选出有斑点的牛.

希望可以帮到你！