Indefinite articles are used when we are referring to an unspecified thing or quantity. We use them when we don’t know (or don’t care) which thing we’re talking about.
Since I don’t know which sheep it was—that is, I don’t know its name, where it’s from, or anything about it—I can’t say the sheep.
How to Use the Indefinite Articles A vs. An
The two indefinite articles in English are a and an. The indefinite article an is used to make pronunciation easier when reading a text aloud. The general rule is to use a when the indefinite article precedes a word beginning with a consonant sound and an when it precedes a word starting with a vowel sound.
1 Use a when the indefinite article comes before a word beginning with a consonant sound: a toy a book a house
2 Use an when the indefinite article comes before a word beginning with a vowel sound:
an operation an idea an apple
It’s important to note that not all nouns that begin with a consonant begin with the consonant sound. Always trust the sound, not the letter when applying the a vs. an rule. Remember, it’s about pronunciation. Consider the examples below:
Likewise, there are some nouns beginning with a consonant that makes a vowel sound. In these cases, the indefinite article a should be used:
Using A vs. An with Acronyms
These same rules apply to acronyms:
Because S sounds like it begins with a vowel (ess), an should be used in front of it.