When we use article the

We use it when :

There is only one of something

The earth goes round the sun and the moon goes round the earth.
What is the longest river in the world?
Paris is the capital of France.
But
we use a/an to say what kind of thing something is
A dog is an animal.
Jill is a really nice person.
The sun is a star.(= one of many stars)

The sky, the sea, the ground, the country, the environment

We looked up at all the stars in the sky.
Would you rather live in a town or in the country?
We must do more to protect the environment.(= the natural world around us)
But
we say space (without „the“) when it mean „space in the universe“
There are millions of stars in space.
BUT: I tried to park my car but the space was too small.

Before same (the same)

Your pullover is the same colour as mine.
These two photographs are the same.

The cinema, the theatre, the bank, the station

I often go to the cinema but I have not been to the theatre for ages.

The radio

BUT: television(without „the“)
I often listen to the radio.
I often watch television.
Can you turn off the television, please?(= the television set)
We heard the news on the radio.
We watched the news on television.
But
There is not a theatre in this town.
I am going to buy a new radio / television (set).
When we mean particular things or people

In general Particular people or things
Children learn a lot from playing. We took the children to the zoo.
All cars have wheels. All the cars in this car park belong to me.
Sugar is not very good for you. Can you pass the sugar, please?

When we talk about a type of animal, musical instruments, machine etc.

The giraffe is the tallest of all animals.
The bicycle is an excelent means of transport.
When was the telephone invented?
Can you play the guitar?
The piano is my favourite instrument.
BUT compare a
I would like to have a guitar.
We saw a giraffe at the zoo.

The + adjective to talk about groups of people

the young the old the elderly
the rich the poor the unemployed the homeless
the sick the disabled the injured the dead

The young = young people, the rich = rich people etc.:
Do you think the rich should pay more taxes to help the poor?
The homeless need more help from the government.

The French are famous for their food.(= the people of France)
Why do the English think they are so wonderful?

the Spanish the Dutch the British the Irish the Welsh
you can not say a French / an English
the + nationality words ending in –ese
The Chinese invented printing.
With other countries, the plural noun ends in –s.
an Italian a Mexican a Scot a Turk
(the) Italians (the) Mexicans (the) Scots (the) Turks
the Germans The Russians the Slovaks

We don`t use “THE:” in :
Continents Africa, Europe, South America
Countries France, Japan, Swityerland
States, regions etc. Texas, Cornwall, Tuscany Central Europe
Islands Corsica, Sicily, Bermuda
Cities, towns etc. Cairo, New York, Madrid
Mountains Everes, Etna, Kilimanjaro
Streets Union Street, ...
Roads Blackrock Road,...
Squares Times Square,...
Parks Hyde Park,...
We use THE in :
Hilton Hotel
National Theatre
Sahara Desert
Atlantic Ocean
White House
Royal Palace

Hotels/restaurants/pubs The Red Lion
Theatres/cinemas The Palace Theatre, The Odeon Cinema
Museums/galleries The British Museum, The Tate Gallery
Other buildings The Empire State Building, The White House
Oceans/seas/canals The Suez Canal, The Indian Ocean
Newspapers The Washington Post, The Financial Times
Organisations The European Union, The BBC

Names with ...of... usually have THE
The Bank of England the Tower of London The Museum of Modern Art
The Houses of Parliament The Great Wall of China The Tropic of Capricorn
The Gulf of Mexico the University of London

Many shops, restaurants, hotels, banks etc, are named after the people who started them. These names end in -`s or –s. We do not use THE with these names :

McDonalds
Harrods (shop in London)
St Paul`s Cathedral

Names of companies, airlines etc. Without THE
Fiat Sony Kodak British Airways IBM