Chapter 1-4-2: Indefinite Articles
The indefinite articles “a” and “an” accompany nouns that have not been particularly identified (as of yet) to the reader or listener. The nouns in question are either:
① Being mentioned for the first time; or
② Imprecise because they are vague, irrelevant, hypothetical or generalized statements.
Knowing When to Use "a" or "an" Before Words
Use the article “a” before words where the “h” is pronounced (house), and use the article “an” before words where the “h” is silent (hour).
Use “an” in front of a noun beginning with a vowel sound. The exception to this is when the word starts with a /y/ sound (yellow, uniform).