as a number: We have only one child. ♦ How much does one pound of apples cost?
as a determiner: He grew roses on one side of his garden, and vegetables on the other. ♦ We’ll meet again one day.
as a pronoun (the plural of this pronoun is ones and the possessive is one’s): It’s a good book, but his last one was better. ♦ One has to trust one’s family. ♦ They are the ones who suffered. (followed by ‘of’): ♦ Sydney is one of the world’s most exciting cities.