ARTCLE

about the indefinite articles, “a” and “an”, and the definite article, “the”.
a/an = indefinite article that refers any noun
the = definite article that refers to a certain or specific noun

For example:

“Let's see a movie” means any  movie. (indefinite)
“Let's see the movie” means a certain movie. (definite) 

“A” or “an”?

“An” is used instead of “a” before a vowel sound. For example:

An artist from the local community painted the wonderful mural at Chemainus.

Indefinite Article: A / AN

Explanation	Example
“A” and “an” are used when it is not clear which thing or person we are talking about.	She was talking to a girl at the store. (This is an unknown girl, so a is used.)
“A” and “an” can be used only with count nouns.	I need a bottle of milk. I need an interesting topic for my speech.

Definite Article: THE

Explanation	Example
“The” is used when it is clear which thing or person we are talking about.	Greg was talking to the girl from New York. (This a specific girl from New York, so the is used.) The sun comes up every day.
“The” is used when there is only one of a particular thing.	London is the capital of England. We went to the most expensive city in the world.
The + adjective “The” is used with some adjectives (without a noun). For example: the young , the old, the sick (The meaning is always plural.)	Do you think that the rich should give money to the poor?
The + nationality adjectives “The” is used with most nationality words which refer to the people of a specific country. For example: the Canadians, the Japanese	The Italians are famous for their delicious food.
Certain nouns use the: names of rivers, oceans and seas: the Mississippi, the Atlantic points on the globe: the Equator, the South  Pole geographical areas: the Middle East, the West, as well as deserts, forests, gulfs, and peninsulas	The Atlantic is a huge, deep, dark ocean. The Middle East is fascinating. The Gulf of Mexico is below Texas. We live near the Saanich Peninsula.

RULES:
1. Use there IS for singular nouns (one item). 2. Use there IS for non-count items(group nouns). 3. Use there ARE for many items (plural nouns).	There is a spider on the wall. There is milk on the floor. There are pencils on my desk

sumber :
http://english-zone.com/verbs/thereisare1.html
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/410-articles-a-an-the.htm