Use of A and An

"A" and "An" are indefinite articles used before singular nouns to indicate that the noun refers to any member of a group or class, rather than a specific one.

"A" is used before words that begin with a consonant sound.
Example: a cat, a house, a book.

"An" is used before words that begin with a vowel sound.
Example: an apple, an egg, an hour.

Example:

• I saw the dog in the park.
  (Here, the word "the" refers to a specific dog that both the speaker and the listener are aware of.)