'The'is different from “a" and “an" in that it refers to a specific object, not a general object.
He sat in a chair. (any chair)
He sat in the chair near the stage. (a specific chair)
Is there a pharmacy near here? (any pharmacy)
I am going to the pharmacy tomorrow. (a specific pharmacy)
It is used with either singular or plural nouns.
I like the painting.
I like the paintings.
It refers to one thing, often unique or individual.
The Earth is the only inhabited planet in the Solar System.
Austin is the capital of Texas.
It therefore, is almost always used with the superlative.
Who is the best student?
This is the cheapest book in the store.
He is the tallest teacher among us.