In English, the term “article” refers to a specific type of adjective that is used to modify nouns. There are two types of articles; definite and indefinite. The indefinite article is “a/an.” The definite article is “the.” Each type is used in specific situations in order to communicate different concepts. Using the wrong article can change the meaning of a sentence.

1. Let’s go see the movie.
2. Let’s go see a movie.

By using the definite article, the first sentence is referring to a specific movie. By using the indefinite article, the second sentence is referring to any movie. In general, the definite article should only be used in conjunction with so-called “non-count” nouns, while the indefinite article should be used with “count” nouns. Count nouns refer to one of many, such as a glass of milk or a piece of pie; non-count nouns can only be one thing, such as the house or the Pacific Ocean.

The indefinite article has two forms, “a” and “an.” Generally, “a” can be used to modify any noun that starts with a consonant, while “an” is used to modify any noun that starts with a vowel. There are notable exceptions. “A” is used with vowels that produce a consonant sound, such as the “y” sound of the word “unique.” “An” is used with words that have a silent “h,” such as “honor.”
Occasionally, the word “historical” can be used in conjunction with “an.”

Here are a few examples:

A cat
An eagle
A house
An hour
A user (vowels making a “y” consonant sound)
A youth
An undertaking

Sometimes, the noun that is being modified will be preceded by a separate adjective. In this case, the form of the indefinite article will be determined by the adjective.

A bald eagle
An eager cat

The definite article is used to indicate a particular object, person or place. It is not always needed in a sentence. However, it is required when the writer is referring to certain proper nouns, especially in connection with geography. Bodies of water traditionally require the definite article:

the Pacific Ocean
the Mississippi River
the Persian Gulf

Geographical regions, including deserts, also require this article:

The Midwest
The Mojave Desert

However, other nouns should never have “the” placed in front of them, including the names of languages, academic subjects or sports.

We studied history.
I speak Russian.
I play chess.

The only exceptions to this rule are when the sentence is referring to a specific object, event or person:

We studied the history of China.
I played the chess tournament champion.
I am familiar with the Russian alphabet.

Finally, a writer may choose to first introduce a concept using the indefinite article and then switch to the definite article. For example:

This paper is based on a study conducted at George Washington University.

The study revealed the following findings…