Rules and applications indefinite articles.

1. CLASS MEMBERS. We use a / an with a name or noun to talk about a member of a class.

1.1. For example:

1.1.1. Maria lives in a nice big house

1.1.1.1. My mom is an engineer

2. TO DESCRIBE. We use a / an when we describe people or things.

2.1. For example:

2.1.1. She is a doctor.

2.1.1.1. My sister is a beautiful woman.

2.1.1.1.1. That is a butterfly.

3. NOT USED A / AN. We do not use a / an with names or nouns that are written in plural or are countless, because a / an means "one".

3.1. For example:

3.1.1. Would you like some milk? (Not ... to milk)

3.1.1.1. We do not use a / an with this single adjective without a noun.

3.1.1.1.1. For example: