Well, if you thought prepositions were “little” words, wait until we consider the part of speech called articles. Articles are similar to adjectives in that they modify nouns, but unlike adjectives, they don’t really describe a noun; they just identify a noun.
Articles are the smallest of the small but still serve an important function. We have three articles in the English language: a, an and the.
The is the definite article, which means it refers to a specific noun in a group.
A or an is the indefinite article, which means it refers to any member of a group. You would use the indefinite article when you aren’t trying to distinguish a particular noun.
Whether you use a or an depends on the noun that follows it. In general, you would use a if the noun begins with a consonant and an if the noun begins with a vowel.
Then, as always in English grammar, there are a few tricky exceptions. If a noun begins with h, you should think about the sound it makes.
And if a noun starts with a vowel, but it makes a y sound, you should use a instead of an.