因为一条裤子是a pair of trousers,裤子用的a pair of 修饰的,所以前后要一致只可以用a longer pair.不用a pair of 修饰的名词单数时就用one,比如The dress is too short. 就可以说You can try this longer one on.连衣裙不可以用a pair of dress,所以只能用one, 不可以用pair.不用a pair of 修饰的可数名词的复数就用ones.比如 I'd like to buy some apples. These or those? The red ones.
做代词时,什么时候用one或ones,什么时候用pair