one是不定代词，只表示数量为一的单数名词，而没有限定，而代词it表示有限定意义的名词。另外，one可以加修饰语，而it不可以，如：
I'd like to borrow a hammer if you have one.
Did you bring the textbook?
No, I didn't bring it.
I lost my mobile hard disk yesterday, so I have to buy a new one.
These boots have worn out. I have to buy some new ones.
I like that picture--I mean the third one from the left.
The guitar is similar to the one that I have.