If you say that someone or something is the one person or thing of a particular kind, you are emphasizing that they are the only person or thing of that kind.

One can be used instead of 'a' to emphasize the following noun.

You can use one instead of 'a' to emphasize the following adjective or expression.

You can use one to refer to the first of two or more things that you are comparing.

You can use one when you have been talking or writing about a group of people or things and you want to say something about a particular member of the group.

You can use one when referring to a time in the past or in the future. For example, if you say that you did something one day, you mean that you did it on a day in the past.

One is the number 1.

You can use expressions such as a hundred and one, a thousand and one, and a million and one to emphasize that you are talking about a large number of things or people.

You can use in one to indicate that something is a single unit, but is made up of several different parts or has several different functions.

One or two means a few.

If you try to get one up on someone, you try to gain an advantage over them.

The one and only can be used in front of the name of an actor, singer, or other famous person when they are being introduced on a show.

You can use for one to emphasize that a particular person is definitely reacting or behaving in a particular way, even if other people are not.

You use one after the other or one after another to say that actions or events happen with very little time between them.

You can use one by one to indicate that people do things or that things happen in sequence, not all at the same time.

You use one or other to refer to one or more things or people in a group, when it does not matter which particular one or ones are thought of or chosen.

You can use one to refer to the first of two or more things that you are comparing.

You can use one or ones instead of a noun when it is clear what type of thing or person you are referring to and you are describing them or giving more information about them.

You use ones to refer to people in general.

You can use one instead of a noun group when you have just mentioned something and you want to describe it or give more information about it.

You can use one when you have been talking or writing about a group of people or things and you want to say something about a particular member of the group.

You use one to make statements about people in general which also apply to themselves. One can be used as the subject or object of a sentence.

You use one in expressions such as 'one of the biggest airports' or 'one of the most experienced players' to indicate that something or someone is bigger or more experienced than most other things or people of the same kind.