When to Use 'A,' 'An,' or 'The'

There are several exceptions, or more complicated situations than the above chart covers. Below we have laid out some of the general and specific rules about using A, AN, and THE.

Remember, in order to use A, AN, and THE properly, you must know whether or not a noun is a Count or Non-Count Noun. (A count noun is the name of something that can be counted: one book, two books, three books. A non-count noun is the name of something that cannot be counted: milk, flour, freedom, justice).

	Use "a" or "an"	Use "the"	Don't Use "a," "an," or "the"
General Rules	Use "a" or "an" with a singular count noun when you mean "one of many," "any," "in general." Bob is a student (one of many students). I like a good movie (one of many movies).	Use "the" with any noun when the meaning is specific; for example, when the noun names the only one (or one) of a kind.  Adam was the first man (the only 'first man'). New York is the largest city in the United States (only one city can be 'the largest'). We live on the earth (the only Earth we know). Have you heard the news (specific news)?	Don't Use "a," "an," or "the" with a non-count noun when you mean "any," "in general." We believe in love (in general). He gave me information (not specific).
	Use "a" or "an" the first time you use a noun in a paragraph. I saw a movie last night. A man ran into the street.	Use "the" the second time you use that same noun in the same paragraph. I saw a movie last night. The movie was entertaining. A man ran into the street. A car hit the man.	Don't Use "a," "an," or "the" with a plural count noun when you mean "some of many things," "any," "in general." Movies are entertaining (some movies; movies in general). She likes men (in general).

For more detailed ....

visit source http://www.gallaudet.edu/tip/english_works/grammar_and_vocabulary/when_to_use_a_an_or_the.html