one it 哪个是同类同物

简单点说one是“同类不同个”, it就是同一个

如 I have no pen and I want to buy one (任一支钢笔,同类）

There only one pen there, I have to buy it ( 就是前面那个）

另外one还可以用作从句的先行词,it不行

The one I like best is gone