1. it是代指原物,知道性别时不能代指人,不能作定语从句先行词.
the one可以代人,也可以代物,常作定语从句先行词.
2. 代物时:
one 泛指,it表示特指.one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 the one 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.