the definite article

 1 used to refer to someone or something that has already been mentioned or is easily understood There were three questions.

 The first two were relatively easy but the third one was hard. 
There was an accident here yesterday. A car hit a tree and the driver was killed. 
The heat was getting to be too much for me. 
The nights are getting longer.

 2 used to refer to someone or something that is the only, normal, or obvious one of their kind 

the Mona Lisa
 the Nile 
the Queen 
What's the matter? 
The phone rang. 
I patted her on the back. 
How's the (= your) baby?

 3 used when explaining which person or thing you mean 

the house at the end of the street 
The people I met there were very friendly
.It was the best day of my life. 
You're the third person to ask me that. 
Friday the thirteenth 
Alexander the Great

 4 used to refer to a thing in general rather than a particular example 

He taught himself to play the violin. 
The dolphin is an intelligent animal. 
They placed the African elephant on their endangered list. 
I heard it on the radio. 
I'm usually out during the day.

MOR INFO :  http://oaadonline.oxfordlearnersdictionaries.com/dictionary/THE