one
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.