The three articles — a, an, the — are a kind of adjective. The is called the definite article because it usually precedes a specific or previously mentioned noun. A and an are called indefinite articles because they are used to refer to something in a less specific manner (an unspecified count noun). These words are also listed among the noun markers or determiners because they are almost invariably followed by a noun (or something else acting as a noun).
Even after you learn all the principles behind the use of these articles, you will find an abundance of situations where choosing the correct article or choosing whether to use one or not will prove chancy.
Icy highways are dangerous.
The icy highways are dangerous.
And both are correct.
The is used with specific nouns. The is required when the noun it refers to represents something that is one of a kind:
The moon circles the earth.
The is required when the noun it refers to represents something in the abstract:
The United States has encouraged the use of the private automobile as opposed to the use of public transit.
The is required when the noun it refers to represents something named earlier in the text.
We use a before singular count-nouns that begin with consonants (a cow, a barn, a sheep).
We use an before singular count-nouns that begin with vowels or vowel-like sounds (an apple, an urban blight, an open door).
Words that begin with an h sound often require an a (as in a horse, a history book, a hotel), but if an h-word begins with an actual vowel sound, use an an (as in an hour, an honor).
We would say a useful device and a union matter because the u of those words actually sounds like yoo (as opposed, say, to the u of an ugly incident).
The same is true of a European and a Euro (because of that consonantal "Yoo" sound).
We would say a once-in-a-lifetime experience or a one-time hero because the words once and one begin with a w sound (as if they were spelled wuntz and won).
Merriam-Webster's Dictionary says that we can use an before an h- word that begins with an unstressed syllable. Thus, we might say an hisTORical moment, but we would say a HIStory book. Many writers would call that an affectation and prefer that we say a historical, but apparently, this choice is a matter of personal taste.
First and subsequent reference: When we first refer to something in written text, we often use an indefinite article to modify it.
A newspaper has an obligation to seek out and tell the truth.
In a subsequent reference to this newspaper, however, we will use the definite article:
There are situations, however, when the newspaper must determine whether the public's safety is jeopardized by knowing the truth.
I'd like a glass of orange juice, please," John said.
I put the glass of juice on the counter already," Sheila replied.
When a modifier appears between the article and the noun, the subsequent article will continue to be indefinite:
I'd like a big glass of orange juice, please," John said.
I put a big glass of juice on the counter already," Sheila replied.
Generic reference: We can refer to something in a generic way by using any of the three articles. We can do the same thing by omitting the article altogether.
A beagle makes a great hunting dog and family companion.
An airedale is sometimes a rather skittish animal.
The golden retriever is a marvelous pet for children.
Irish setters are not the highly intelligent animals they used to be.
The difference between the generic indefinite pronoun and the normal indefinite pronoun is that the latter refers to any of that class ("I want to buy a beagle, and any old beagle will do.") whereas the former (see beagle sentence) refers to all members of that class.
Proper nouns: We use the definite article with certain kinds of proper nouns:
Geographical places: the Sound, the Sea of Japan, the Mississippi, the West, the Smokies, the Sahara (but often not when the main part of the proper noun seems to be modified by an earlier attributive noun or adjective: We went swimming at the Ocean Park)
Pluralized names (geographic, family, teams): the Netherlands, the Bahamas, the Hamptons, the Johnsons, the New England Patriots
Public institutions/facilities/groups: the Wadsworth Atheneum, the Sheraton, the House, the Presbyterian Church
Newspapers: the Hartford Courant, the Times
Nouns followed by a prepositional phrase beginning with "of": the leader of the gang, the president of our club
Abstract nouns: Abstract nouns - the names of things that are not tangible - are sometimes used with articles, sometimes not:
The storm upset my peace of mind. He was missing just one thing: peace of mind.
Injustice was widespread within the judicial system itself. He implored the judge to correct the injustice.
Her body was racked with grief. It was a grief he had never felt before.