One & Ones 

We use one (singular) and ones (plural) to avoid unnecessary repetition .

• 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 .
• 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:

• You can borrow a book. Which one do you want?
• There are lots of books here. Which ones are yours?