one表示泛指，that和it 表示特指。that与所指名词为同类，但不是同一个，而it 与所指名词为同一个。
I can't find my hat. I think I must buy one. (不定)
我找不到我的帽子了。我想我该去买一顶。
The hat you bought is bigger than that I bought. (同类但不同个)
你买的那顶帽子比我买的大。
I can't find my hat. I don' t know where I put it. ( 同一物)
我找不到我的帽子。我不知道我把它放在哪了。