ARTICLES (A, AN, THE) [art or ^]

Maria bought a new car.  
It's a brand new day.

• occur in some set expressions (idioms):

It's the same story (lesson, way, message).  
One of the musicians can play the flute.

• precede most countries that end in a plural  noun and countries made up of states/provinces/other countries:

the United States 
the Russian Federation

• precede the names of oceans, seas, gulfs and rivers:

The Pacific Ocean 
The Mediterranean Sea 
The Gulf of Mexico  
The Mississippi River

• precede specific nouns that are easily identifiable because they refer to a unique object or are modified by superlative adjectives:

The sky is blue and  the sun is shining. 
The computer that you are using  needs more software. 
She's studying the Indonesian language. 
Is this the right answer or the wrong answer?

• occur in expressions of time and ordinal numbers:
  

She is  the first (second, third, last) person to get the right answer. 
 

• have already been introduced (second mention) or are defined in the context of the sentence:

Note the first and second mention from "The Fox and The Crow," one of Aesop's fables:

One day, a hungry fox was walking along a path when he saw a crow sitting on a branch of a tree, eating a piece of cheese. The fox wanted the cheese, so he said to the crow, "Mister Crow, you have such a beautiful voice. Would you please sing a song for me?" Flattered, the crow opened his mouth to sing the song, and the cheese fell right into the mouth of the clever fox.