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,...