Indefinite articles:
These are “a” and “an.” Indefinite articles are generally used when the speaker is referring to a non-specific item.
e.g. I would like a coffee. (Any coffee)
We use “an” before a word beginning with a vowel (a, e, i, o or u) sound and “a” before a word beginning with a consonant (non-vowel) sound.
e.g. a car, an umbrella
Note that it is the sound a word begins with when spoken that determines whether “a” or “an” is required, so it is best to say a word aloud to decide which one to use.
e.g. even though “unicorn” is spelled with an initial “u” (a vowel), it is pronounced “you-ni-corn,” so we say “a unicorn.”
Similarly, although “hour” is spelled with an initial “h” (a consonant), this is silent (i.e. “our”), so we would say “an hour.”
The definite article:
This is “the,” and it is generally used to refer to something specific.
e.g. Would you bring me the bill? (Specifically my bill and not someone else’s)
For a more comprehensive guide to using articles, including situations in which they are not used (the zero article), I would recommend A Short Article on Articles by David Appleyard
Photo credit: kate mccarthy (Creative Commons)