Once you have decided that an article is necessary, you must choose between an indefinite article (a and an) the definite article (the). In general, you should use the definite article if the noun is unique or known to the reader and an indefinite article if the noun is one of a group or new to the reader.
Use an indefinite article (a or an) with singular countable nouns to refer to an arbitrary member of a larger group or to introduce a member of the larger group for the first time. The indefinite article always implies that other examples of the noun exist.
Piece of string one light-year long would wrap around the equator of the
Earth 236 million times.
[Piece is singular and countable, so an
article (definite or indefinite) must be used.]
The piece of string one light-year long would wrap
around the equator of the Earth 236 million times. [The definite article
implies that only one piece of string exists or that the author already mentioned
this particular piece of string before.]
A piece of string one light-year long would wrap around
the equator of the Earth 236 million times. [The indefinite article
implies that the author is introducing this particular piece of string, yet other
pieces of string exist.]
--Andrew Fraknoi, "The Universe: An Introduction"
Never use an indefinite article with a plural countable noun or with an uncountable noun.
Unlike a stars, a planets do not
shine under their own power, but simply reflect the light of their star.
Unlike stars, planets do not shine under their own power, but simply
reflect the light of their star.
--Andrew Fraknoi, "The Universe: An Introduction"
Use the definite article (the) with singular countable nouns to suggest that the noun is unique (its group consists of only one member) or to refer to a noun that has been previously introduced. The definite article used with a singular countable noun usually implies that only one example of the noun exists or is relevant to the situation.
The Earth is one of nine planets circling
the star we call the Sun.
--Andrew Fraknoi, "The Universe: An Introduction"
You can also use the definite article with a singular countable noun to refer to the entire class that the noun belongs to. This is often done with species of animals, inventions, or musical instruments:
The honey possum of Australia is the only mammal that
lives exclusively on nectar.
--"Take It or Leave It," Valley Comic News
Use the definite article with plural countable nouns or uncountable nouns to refer to the entire category in existence or to a specific subset of the category as defined by the context.
When particles of matter meet with and annihilate their corresponding
particles of antimatter, the energy produced comes out purely
in the form of gamma rays. [The author used the definite article with the
uncountable noun energy to indicate that he was referring to only the
subset of energy that is produced when these particles collide.]
--Harding E. Smith, "Quasars and Active Galaxies"
Do not use the definite article when referring to uncountable nouns or plural countable nouns in a general sense.
The discovery of the pulsars and the efficient way in
which the pulsars supply energy to the
supernova remnants like the Crab Nebula has led to the suggestion that
the rapidly rotating, magnetized, massive objects might also
power the quasars.
The discovery of pulsars and the efficient way in which pulsars supply
energy to supernova remnants like the Crab Nebula has led to the suggestion that
rapidly rotating, magnetized, massive objects might also power quasars.
--Harding E. Smith, "Quasars and Active Galaxies"
Use the definite article whenever you modify a noun with a superlative adjective (using most or least with the modifier or adding -est to it), with the word same, or with an ordinal number (first, fourth).
Most common cause of coughing is stress.
The most common cause of coughing is stress.
--"Take It or Leave It," Valley Comic News
First steam locomotive in regular service was in England in 1814.
The first steam locomotive in regular service was in
England in 1814.
--"Take It or Leave It," Valley Comic News
At same temperature and at same level, moist air weighs less than dry
air.
At a same temperature and at a same
level, moist air weighs less than dry air.
At the same temperature and at the
same level, moist air weighs less than dry air.
--C. Donald Ahrens, Meteorology Today
Always use the definite article before the names of groups of years, such as decades:
When fax machines were invented in 1960s, it took an average of 6 minutes
to send a one-page document.
When fax machines were invented in the 1960s, it took
an average of 6 minutes to send a one-page document.
--Les Krantz, America by the Numbers
