The indefinite articles a and an are used to indicate a non-specific thing, or any instance of a thing. Whether to use a or an depends not on spelling, but on the sound of the word that comes immediately after the article. The article a is used before words beginning with a consonant sound, while the article an is used before words beginning with a vowel sound.

Use a before a word that begins with a w-sound or a y-sound, even if the word itself begins with a vowel, like united and one.

Several agencies suspected a one-armed man of the crime, and they made a united effort to catch him.

Use an before a word that begins with a vowel sound, even if the word itself begins with a consonant, like honest.

She gave an honest answer.

Note that the a and an distinction applies to the very next word in the sentence, not to the word that the a or an actually modifies. If the next word is in parentheses, however, the article can ‘match’ the first word after the parentheses.

This line should be read as a, hopefully intriguing, example of an, admittedly unlikely, sentence. (“a” modifies “example”; “an” modifies “sentence”)

Use the indefinite article the first time that a (indefinite) general thing is referred to. (“a” modifies “thing”)

The definite article used before an acronym should also depend on how the acronym would be pronounced out loud.

According to an SVA official… According to a USDA official…