The definite article (THE)

USE OF THE DEFINITE ARTICLE

 
The definite article THE is used:

•  with a singular and plural nouns. 
• countable and noncountable nouns
• when the speaker and the listener know the thing or idea already

Ex: The cat is old, but the the dog is just a puppy.  
Ex: I´m going to the supermarket. Do you want anything?

RULES:

1. Before oceans , rivers, hotels, theaters, museums and newspapers

ex: The Atlantic, The Metropolitan museum, The Washington Times, The Ritz
 
2.   If there is only one of something

ex: The sun, The president, The government

3. With superlative adjectives

ex: He is the richest man in the world,  Jane´s the oldest in the class