The words 'A', 'An' and 'The' are called articles. The articles are used before nouns.
'A' is used before the nouns that begin with a consonant.
- A boy
- A bridge
- A car
- A rocket
- A horse
'An' is used before the nouns that begin with a vowel.
- An apple
- An instrument
- An idea
- An orange
- An Umbrella