Articles

The words a, an, and the are generally called articles and sometimes classed as a separate part of speech. In function, however, they can be grouped with the demonstrative adjectives that are used to point things out rather than describe them.

Definite Article

The is called the definite article because it points out a particular object or class.
This is the book I was talking about.
The dodo bird is extinct.

Indefinite Article

but not any particular specimen.
 Here are some examples : a book, a dog, a lawn mower

The indefinite article has two forms:
A is used before words beginning with a consonant sound or an aspirated h:
a car, a lamb, a hope, a habit, a hotel

An is used before words beginning with a vowel sound:
an ape, an image, an untruth, an honorable man