Indefinite Article (a/an):
"A" is used before words that begin with a consonant sound, e.g., "a car," "a book."
"An" is used before words that begin with a vowel sound, e.g., "an apple," "an hour."
Definite Article (the):
"The" is used before singular or plural nouns when the speaker or writer assumes the listener or reader knows exactly which thing or things they are referring to, e.g., "the book," "the car."
It is also used before unique or specific nouns, e.g., "the sun," "the President."