one表示泛指,that和it 表示特指。that與所指名詞為同類,但不是同一個,而it 與所指名詞為同一個。
I can't find my hat. I think I must buy one. (不定)
我找不到我的帽子了。我想我該去買一頂。
The hat you bought is bigger than that I bought. (同類但不同個)
你買的那頂帽子比我買的大。
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。