A, an, and the are demonstrative adjectives, but they are also called articles. They are used to point things out vs. describe things, which makes them a bit different from most adjectives.
There are two types of articles: definite and indefinite.
Definite Article: The is a definite article because it points to a particular object or class, such as:
- This is the song I was telling you about.
- The red-eared slider is my favorite turtle.
Indefinite Article: A and an are called indefinite articles because they point out an object in general vs. a particular one. A is used with words beginning with a consonant sound or an aspirated h. An is used with words beginning with a vowel sound. For example:
- A dog ran down the street.
- We need to book a hotel room for our trip.
- In a trial, it is best to have an honorable judge.
- An apple a day is supposed to be healthy for you.