The Use of Articles

English has two articles: the and a/an.
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.

the = definite article                a/an = indefinite article 

For example, if I say, "Let's read the book," I mean a specific book.
If I say, "Let's read a book," I mean any book rather than a specific book.

Here's another way to explain it: The is used to refer to a specific or particular member of a group. For example, "I just saw the most popular movie of the year." There are many movies, but only one particular movie is the most popular. Therefore, we use the. 

"A/an" is used to refer to a non-specific or non-particular member of the group. For example, "I would like to go see a movie." Here, we're not talking about a specific movie. We're talking about any movie. There are many movies, and I want to see any movie. I don't have a specific one in mind.

Let's take a closer look at the use of the articles in English.

General rules:

1.  a/an before singular countable nouns mentioned for the first time
2. a/an before some expressions of quantity
3.  the before proper nouns (eg rivers and mountain ranges)
4.  the before superlative adjectives
5.  the before singular countable nouns when we have a specific thing in mind
6. no article before countries, languages, cities, etc
7. no article before people or things in general

Ex 1         Ex 2