The Definite & Indefinite Articles

"The" is called the definite article. It is used to designate a specific thing or person known to the speaker/writer and the listener/writer. "A" or "an" is called the indefinite article; it is used to designate an unidentified thing or person.

This page deals with the details on which article to use.
An overview can be found in this flowchart.

Use "the" Use "a" or "an"
... with most nouns whose specific identity is known to the listener/reader because of one of the following reasons:
  1. The noun has been mentioned before.

    I have a stamp album. The album contains stamps from more than 20 countries.

    The indefinite article "a" is used to introduce the noun "album" for the first time. When the noun is mentioned again, the definite article "the" is used since readers now know which stamp album is being discussed.

  2. A phrase or clause identifies the noun.

    Who's the beautiful lady over there with David?

    The phrase "over there with David" identifies the specific lady.

  3. The context or situation makes the noun's identity clear.

    Please close the door.

    Both the speaker and listener know which door is meant ... the door that is open.

  4. The noun describes a unique person, place or thing.

    I visited the Great Wall of China.

    There is only one Great Wall of China so its identity is clear.

  5. A superlative makes the noun's identity specific.

    Mary is the tallest girl in her school.

    Usually only one girl can be the "tallest" girl so it is clear which one we are talking about.

    For the same reason, the definite article "the" is used with nouns with these adjectives: "first", "last", "next", "same" and "only".
 ... with singular countable nouns whose specific identity is not known to the listener/reader because ...
  1. The noun is being introduced for the first time to the listener/reader.

    See the entry in the left-hand column.

  2. The speaker/writer is referring to a person or thing in general.

    A nurse should be in the profession because of compassion not money.

    The indefinite article usually means "one among many but it can mean "any one" as in the above sentence where "a" nurse means "any" nurse.

  3. The noun's specific identity is not known even to the speaker/writer.

    We are looking for an apartment within the central business district.

    Generally, "a" is used before a consonant while "an" is used before a vowel (a,e,i,o,u).

    Exceptions:

    • use "a" before a consonant sound:
      a one-dollar coin,
      a European country,
      a uniform,
      a university

    • use "an" before a vowel sound:
      an honest man,
      an hour,
      an heir

When not to use the articles ...