ones 代替可数名词复数形式, 泛指
the ones也代替可数名词复数形式,特指,在没有定语修饰ones 的情况,相当于those
e.g.
　　When shopping, we always choose the fish that are frresh, the ones/those with bright eyes.
Hard beds are healthier than soft ones. 硬板床比软床有利于健康.
Green apples often taste better than red ones. 青苹果往往比红苹果好吃.