Articles are little words that you use just about every time you speak—but there are only three of them: “a,” “an,” and “the.” They allow you to refer to a specific noun or a noun in the general sense.
When indefinite articles are used before a noun (such as a ball), they do not refer to a specific noun (like the soccer ball) but a noun in general (any ball—maybe a tennis ball or a football or even a baseball).
So if you tell your friend to grab a ball, she might bring back a tennis ball or a football or even a baseball because you were not specific about which ball you wanted.
“A” vs. “An”
Use “a” before a singular noun that begins with a consonant.
I have a plum.
Use “an” before a singular noun that begins with a vowel.
I saw an apple.
The intent behind this rule is to make your sentences a little easier to say aloud. (Vocalizing “a apple” takes a lot of unnecessary enunciation skill).
“The” is a definite article. Use it when you are referring to a specific noun.
The new student was late for class.
I’m not talking about just any student, but the student who is new to school (and apparently tardy).
There are special circumstances when nouns are not preceded by any article. This occasionally happens when you refer to something in general or with mass/non-count nouns (for example, “water” or “animals”).
Students spend a lot of money on textbooks.
I’m not referring to any specific group of students (not transfer students or honor students or freshmen), just students in general.