2 used for referring to a single person or thing when there are others of the same type
I bought three T-shirts. Do you want one?
One passenger said she had been waiting for 13 hours.
I heard of one case where a 12-year-old boy had been sent to prison.
one of: Ours was one of the top four boats in the competition.
One of the buildings was badly damaged in the fire.
I met one of her brothers when he was here on business.
just the one: “How many books did she borrow?” “Just the one.”