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:

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 peakwere 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.(= Joe 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

Indefinite Article: "a" and "an"

We use the indefinite article, a/an, with count nouns when the hearer/reader does not know exactly which one we are referring to:

Police are searching for a 14 year-old girl.

We also use it to show the person or thing is one of a group:

She is a pupil at London Road School.

We do not use an indefinite article with plural nouns and uncount nouns:

She was wearing blue shoes. (= plural noun)
She has short blonde hair. (= uncount noun)

We use a/an to say what someone is or what job they do:

My brother is a doctor.
George is a student.

We use a/an with a singular noun to say something about all things of that kind:

A man needs friends. (= All men need friends)
A dog likes to eat meat. (= All dogs like to eat meat)

The 'Zero' Article

To talk about plural and uncountable nouns or when talking about things in general:

I'm terrified of heights
I'm into drum and bass.
I hate cheese.

Before countries, towns, streets, languages and single mountains:

I'm from China.
I've climbed Mount Everest.
She speaks French.

Before some places and with some forms of transport:

I live at home with my parents.
I came here by car.
He goes to work by bus.

In exclamations with what + uncountable noun:

What beautiful weather!
What loud music!
What disgusting food!

Sources of information was used for preparing this article:

1. British Council
2. BBC Learning English