Ways to use Definite Articles
THE
1. Definite Articles DO NOT change when talking about Boys and Girls or NUMBERS of the noun they are referring to. 'The' is used to talk about a specific person or thing.
For example: The boy went to the house.
The woman was going to the store.
The children were on the bus.
2. The is used when something has already been mentioned.
For example: A boy and a girl went to
the store. 
The boy bought
a candy bar and the girl bought a carton of
milk.
3. The is used when both the listener and speaker know what is being talked about in the sentence or when we can identify a person or object.
For example: "Where's the library?" "It is over
by the science building."
4. The is used when referring to one of a kind objects, adjectives referring to a group of people, geographical areas and oceans, and when talking about decades.
For example: In the early 1800's the Irish worked until the sun went down in the potato fields.
5. You don't use the definite article before meals, languages, sicknesses, and sports.
For example: He has asthma.
I went to lunch at noon.
http://www.i-claudius.com/esl/definart.html
http://www.edufind.com/english/grammar/DETERMINERS2a.cfm
http://stufun.com/articles/definite.php3
