article

We have three articles in the English language: a, an, and the. The words a and an are indefinite articles, the word is a definite article.

Use an when the word modified begins with a vowel or vowel sound, as in an apple or an hour.

Use a when the word modified begins with a consonant or consonant sound, as in a bicycle or a university.

When a or an modifies an acronym, use an when the spoken acronym begins with a vowel sound, as in an FDA hearing.

The differences between a-an and the, and the issue of when to omit articles are discussed in the section on Adjectives in Parts of Speech on Grammar.com. Click here for that discussion.