The words a, an, and the are articles. Articles are used with nouns to specify whether the noun refers to any item named by the noun or to one specific item.
The articles a and an are only used with nouns that name things that can be counted.
Days, apples, and friends can be counted. You may have one day, two apples, and three friends. Nouns that name things that can be counted are called count nouns.
Soup, water, and luggage cannot be counted. You don’t have one soup, though you may have one bowl of soup, one glass of water, and too much luggage. Nouns that name things that cannot be counted are called mass nouns.
The articles a and an mean “one,” so a and an can be used only with count nouns. They are not used with mass nouns.
The articles a and an make a noun nonspecific; that is, they make it refer to any of the things it names. For example, consider the following sentence:
I need a wrench to fix my bike.
The sentence uses a to indicate that any wrench will do.
The article the makes a noun specific; that is, it makes the noun refer to just one of the things it names. For example, consider the following sentence:
Hand me the biggest wrench.
The sentence uses the to indicate that only one wrench will do.
The article a is used before words that begin with a consonant sound, and an is used before words that begin with a vowel sound.
Notice that the distinction between a and an is based on sounds, not on letters. Do not simply use a before all words beginning with consonant letters and an before all words beginning with vowel letters.
The following examples correctly use a and an:
an SAT score
What is the basis for choosing between a and an?