Definite Article: THE

1 - We use THE:

* With plural nouns
The books are expensive.

* With singular nouns when it is the second time we mention it
There is a car in the street… The car is blue.

* When the noun is followed by a clause or a phrase
The boy that you met yesterday is my brother.
Look at the man next to the horse!

* Before names of musical instruments
I can play the piano and the guitar.

* Before nouns which are unique
The moon
The sun
The earth
The sky
The North Pole
The Parthenon

* Before the words: morning / afternoon / evening.
I watch TV in the evening.

* Before names of rivers, seas, oceans, chains of mountains, plural names of countries.
The Nile
The Mediterranean
The Pacific
The Andes
The Sahara
The USA
The Netherlands

* Before nationalities
The Japanese
The British

* Before names of families
The Smiths

2 - We cant’ use THE:

* Before proper nouns
Mary is very intelligent.

* Before plural nouns when we talk about them in general
Children like playing. (i.e. children in general)

* Before names of sports
I play tennis and golf.

* Before the word home.
She is at home
They went home
He arrived home at 8:00.

* Before certain nouns when they are used for their primary purpose.
We go to school / university. (to study)
We go to church. (to pray)
She is in hospital (She is a patient)
Go to bed! (to sleep)
She is at work. She went to work. (She is a worker)
They went to prison. (They are prisoners)

Definite and Indefinite articles.(Exercises) Try these great links:

http://www.englisch-hilfen.de/en/exercises/artikel_a.htm

http://www.englisch-hilfen.de/en/exercises/artikel_a2.htm

http://www.englisch-hilfen.de/en/exercises/articles/index.php

http://perso.wanadoo.es/autoenglish/gr.an.i.htm

http://www.englisch-hilfen.de/en/exercises/artikel_the.htm