The is used to refer to specific or particular nouns; a/an is used to modify non-specific or non-particular nouns. We call the the definite article and a/an the indefinite article. For example, if I say, "Let's read the book," I mean a specific book.
How do you use the word the?
Use "the" with any noun when the meaning is specific; for example, when the noun names the only one (or one) of a kind. Adam was the first man (the only 'first man'). New York is the largest city in the United States (only one city can be 'the largest'). We live on the earth (the only Earth we know).
