The definite article (the) modifies a noun that describes a particular thing.

The indefinite article (a or an) modifies a noun that describes a generality.

Compare the following:

  • An apple a day is good for your health. (general; any apple)
  • The apple in my hand comes from Prince Edward County. (a particular apple)
  • A degree or diploma is a formula for success. (general; there are many degrees and many formulas for success)
  • E = mc² is the formula that made Einstein famous. (a particular formula)

The definite article (the), meaning only one, modifies a noun that describes a particular person, place, or thing:

  • Agnes Macphail was the first woman elected to the House of Commons of Canada. (There is only one woman who was first to be elected to it, and there is only one House of Commons of Canada).
  • The sun is the centre of our solar system. (There is only one sun in our solar system, and the solar system has only one centre.)

The indefinite article (a or an), meaning one of many, appears before a singular noun or adjective:

  • She is a scientist.
  • It is a difficult experiment.
  • Life is a mystery.
  • That is a revolutionary painting.

Idiomatically, “a may also have the meaning of “one.” Note that we rarely use “one” to modify a singular noun; “a” is customary, e.g., There is a mosquito in this room.

No article precedes a plural countable noun or an uncountable noun. When an article does not precede a noun, as in the examples below, we refer to it as zero (Ø) article:

  • Ø Elephants are highly social, intelligent beings.
  • Ø Economic inequality is an important subject to study.
  • They are Ø friends of mine.
  • Ø Music is diverse.
  • Ø Courage is essential for a good life.