Articles are words that specify whether a noun is singular or plural and whether it is definite (specific) or indefinite (general).
* "a" is used before singular indefinite nouns, like "a cat" or "a book".
* "an" is used before singular indefinite nouns that start with a vowel sound, like "an apple" or "an hour".
* "the" is used before singular or plural definite nouns, like "the cat" or "the books".