We use one (singular) and ones (plural) to avoid unnecessary repetition.
我們用one（單數(shù)）和ones（復(fù)數(shù)）避免不必要的重復(fù)。

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.
我的褲子破了，我需要一些新褲子。

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:
通常我們把one和ones放在which后提問(wèn)：

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

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