I'd like to borrow a hammer if you have one.
We needn't have brought a camera. He has one.
one也可以被壹些修飾語所修飾，比如說：
The umbrella is broken. I have to buy a new one.
The socks have worn out. I have to buy some new ones.
The painting is good. I mean the third one from the left.
The guitar is similar to the one I have.
one只用於代替可數名詞，不能代替不可數名詞，比如：
She likes red wine better than white.