Use of "a" and "an"

The words "a" and "an" are indefinite articles used in English grammar to indicate a singular noun or a noun that starts with a consonant or vowel sound.
"A" is used before:
Nouns that start with a consonant sound: a cat, a dog, a house
Nouns that start with a vowel that sounds like a consonant: a union, a European
"An" is used before:
Nouns that start with a vowel sound: an apple, an elephant, an hour
Nouns that start with a silent "h": an hour, an heir
Remember, the choice between "a" and "an" depends on the sound of the noun, not its spelling.
For example:
"A unicorn" (starts with a consonant sound)
"An umbrella" (starts with a vowel sound)
I hope this helps clarify the definition and use of "a" and "an"!