Articles in English are very important, as we use them all the time. The three articles in English are a, an, and the. Here are some basic rules for understanding how to use these articles:
The is the definite article. It is used before singular or plural nouns that are specific or particular. Here are some examples:
"The girl who lives next door to me is really cute." This refers to a particular girl: the girl who lives next door. "The president is a busy man." There is only one president, so we are referring to a specific noun here. "I love the books you gave me." Again, we're talking about particular books, the ones you gave me.
A/an are the indefinite articles. We use a/an before general, non-specific nouns or to indicate membership in a group. A/an can only be used with countable, singular nouns. Here are some examples of how to use a/an:
"Let's go see a movie tonight." We aren't talking about a specific movie, as we haven't said which movie we want to see. "Cassie is an actor." Cassie belongs to a group: actors. We use "an" instead of "a" because "actor" begins with a vowel. "I hope I get a car for my birthday." This refers to any car. I don't know which car yet because I haven't gotten the car.
Finally, all articles in English are invariable, meaning that they do not change if the noun is singular or plural, masculine or feminine. There are no other forms of the, an, or a.