definite article: the

The definite article the is the most frequent word in English.
We use the definite article in front of a noun when we believe the hearer/reader knows exactly what we are referring to.
• because there is only one:

The Pope is visiting Russia.
The moon is very bright tonight.
The Shah of Iran was deposed in 1979.

This is why we use the definite article with a superlative adjective:

He is the tallest boy in the class.
It is the oldest building in the town.

• because there is only one in that place or in those surroundings:

We live in a small village next to the church.	=	(the church in our village)
Dad, can I borrow the car?	=	(the car that belongs to our family)
When we stayed at my grandmother’s house we went to the beach every day.	=	(the beach near my grandmother’s house)
Look at the boy in the blue shirt over there.	=	(the boy I am pointing at)

• because we have already mentioned it:

A woman who fell 10 metres from High Peak was lifted to safety by a helicopter. The woman fell while climbing.
The rescue is the latest in a series of incidents on High Peak. In January last year two men walking on the peak were killed in a fall.

We also use the definite article:
• to say something about all the things referred to by a noun:

The wolf is not really a dangerous animal (= Wolves are not really dangerous animals)
The kangaroo is found only in Australia (= Kangaroos are found only in Australia)
The heart pumps blood around the body. (= Hearts pump blood around bodies)

We use the definite article in this way to talk about musical instruments:

Joe plays the piano really well.(= George can play any piano)
She is learning the guitar.(= She is learning to play any guitar)

• to refer to a system or service:

How long does it take on the train.
I heard it on the radio.
You should tell the police.

• With adjectives like rich, poor, elderly, unemployed to talk about groups of people:

Life can be very hard for the poor.
I think the rich should pay more taxes.
She works for a group to help the disabled.

The definite article with names:

We do not normally use the definite article with names:

William Shakespeare wrote Hamlet.
Paris is the capital of France.
Iran is in Asia.

But we do use the definite article with:
• countries whose names include words like kingdom, states or republic:

the United Kingdom; the kingdom of Nepal; the United States; the People’s Republic of China.

• countries which have plural nouns as their names:

the Netherlands; the Philippines

• geographical features, such as mountain ranges, groups of islands, rivers, seas, oceans and canals:

the Himalayas; the Canaries; the Atlantic; the Atlantic Ocean; the Amazon; the Panama Canal.

• newspapers:

The Times; The Washington Post

• well known buildings or works of art:

the Empire State Building; the Taj Mahal; the Mona Lisa; the Sunflowers

• organisations:

the United Nations; the Seamen’s Union

• hotels, pubs and restaurants*:

the Ritz; the Ritz Hotel; the King’s Head; the Déjà Vu

*Note: We do not use the definite article if the name of the hotel or restaurant is the name of the owner, e.g.,Brown’s; Brown’s Hotel; Morel’s; Morel’s Restaurant, etc.

• families:

the Obamas; the Jacksons

SOURCE:  http://learnenglish.britishcouncil.org/en/english-grammar/determiners-and-quantifiers/definite-article