Article adjectives come before a noun and describe them.
Prepare to have your minds blown, Shmoopers. The definite article, the, and the indefinite articles, a and an, are article adjectives.
Ka-blow! (That was your mind exploding.)
Use the when you want to refer to a specific thing. Use a or an when you don't need to identify a particular item.
"A sparrow built a cozy nest out of twigs and cigarette butts in the tree outside of Jen's bedroom window."
Since we're not talking about a specific sparrow here, we use the indefinite article a. We are referring to a specific tree, though, so we use the definite article the to refer to the tree outside of Jen's window. You know, the one filled with birds and garbage.
"Have you seen the giant foam finger I bought at the Lakers game last weekend? I can't seem to find it."
Nothing expresses your unwavering basketball fandom like an oversized, spongy digit. In this example, the definite article the is used to describe both the foam finger and the Lakers game because the speaker is referring to a specific foam finger (the one purchased at the Lakers game) and a specific Lakers game (the one last weekend).
"Todd claims a ghost lives in his underwear drawer. The ghost looks just like his late hamster, Giggles."
When the ghost is referred to a second time, however, we use the definite article the because we're referring to the individual ghost that has already been mentioned.