Articles in English are invariable. That is to say, they do not change according to the gender or number of the noun they refer to. THE is called a definite article. "Definite" means "specific". We use THE when we are talking about something which is already known to the listener or which has been previously mentioned, introduced, or discussed.
THE is used:
To refer to something which has already been mentioned.
A boy met a girl in the street.
The boy loved the girl at first sight.
When both the speaker and listener know what is being talked about, even if it has not been mentioned before.
"Where's the bathroom?"
"It's on the first floor."
In sentences or clauses where we define or identify a particular person or object.
The man who wrote this book is famous.
"Which car did you scratch?"
"The red one."
My house is the one with a blue door.
To refer to objects we regard as unique.
Before superlatives and ordinal numbers.
The highest building in town.
The first page of the document.
The last chapter of the book.
With adjectives, to refer to a whole group of people.
With names of geographical areas, rivers, oceans and seas.
The Atlantic Ocean
With names of countries in the plural, mountain ranges, regions and groups of islands.
The United States of America
The Rocky Mountains
The Middle East
The British Isles
With decades, or groups of years.
she grew up in the seventies.
With musical instruments*:
Ralph can play the piano and the violin too . *(in American English no article)
With hotels, cinemas and theatres.
I often go and see plays at the Garrick Theatre.
I like spending the weekend at the Ritz.