我们用one（单数）和ones（复数）避免不必要的重复。

See those two girls? Helen is the tall one and Jane is the short one.

Which is your car, the red one or the blue one?

My trousers are torn. I need some new ones.

我的裤子破了，我需要一些新裤子。

See those two girls. Helen is the one on the left.

Let’s look at the photographs. The ones you took in Paris.

We often use them after Which... in questions:

通常我们把one和ones放在which后提问：

You can borrow a book. Which one do you want?

There are lots of books here. Which ones are yours?