When one talks about a thing in general, the + a singular countable noun is used. Or, one canuse the plural noun without an article.
Examples:
The dog is my favourite domestic animal.
Dogs are my favourite domestic animals.
For talking about a type of machine or invention, the + a singular countable noun is used.
Examples:
The computer has changed our lives.
When was the MRI scan first used?
The is also used for talking about musical instruments.
Examples:
My son is learning to play the piano.
"The" + Adjective
The is used with adjectives (without a noun), when talking about something in general.
.
Examples:
The rich have taken over this part of the town.
He is always helping out with charities for the poor.
These expressions are always plural. One cannot say an injured or a young.
Examples:
The injured have all been taken to the General Hospital.
Nowadays the young expect instant gratification.
"The" + Nationalities
The can be used with some nationality adjectives, to talk about the people who live in a certain country.
Examples:
The French are a very proud nation.
The English aren’t very good at learning other languages, are they?
The can be used with the following nationalities:
The British, the Welsh, the Spanish, the Dutch The English, the Irish, the French, and the Swiss. Also with nationalities that end in -ese, like Chinese.
With other nationalities, the plural noun ending in -s has to be used.
Examples:
The Swiss are well-known for their cheese-making.
The Dutch had many colonies in the East.
The Japanese are often mocked for their love of taking photos.
Russians are frequently seen in the Mediterranean countries nowadays.