Articles
A definite article such as "the" is used to refer to a specific instance of the noun, often already mentioned in the context or easy to identify. Definite articles are slightly different from demonstratives, which often indicate the location of nouns with respect to the speaker and audience.

* "Let's look for a good restaurant."
* "What about the restaurant we ate at last week?"
* "That restaurant was terrible. What about this one on the corner here?"

An indefinite article such as "a" or "an" is used to refer to a generic instance of the noun. Note that "a" is used when preceding a noun beginning with a consonant sound, whereas "an" is used when preceding a noun beginning with a vowel sound.

* You should have a drink.
* That building is a university.
* They are being an annoyance.
* He is an heir to the throne.