an

Like its cousin a, the indefinite article an is an adjective.

The first sound of the word that follows determines which you should use.

A vowel sound (whether the letter is a vowel or not) calls for an (an apple, an honor, an onion).

Nouns for things you can count often use an to indicate one (an airplane, an egg), but things you can’t count don’t need an (electricity, ozone).

An can also precede singular words that express a quantity (an inch of rain, an acre of land).