The definite article the is used before common nouns used in a particular sense.
The book on the table is yours.
The answer is correct.
The Moon is full.
The girl you met is my sister.
In narration when a person or thing is mentioned for the first time the indefinite article is used, but thereafter the definite article is placed before the noun, because it is now used in a particular sense.
My sister give me a book. -> The book is very interesting.
I saw some beautiful shoes. -> The shoes are not expensive.
The definite article also is used before ordinal numbers.
Examples : The definite article before numbers
Thursday is the fourth day of the week.
Nowadays in The twentieth first century.
Before common nouns to indicate the whole class.
Examples : The definite article indicate whole class
The kangaroo is a marsupial.
The fly is an insect.
The cow give us milk.
Before the Superlative
Examples : The definite article before the Superlative
She is the prettiest girl in the class.
This is the best game that I ever see.
I am the best among us.
The best story ever written.
The definite article before unique things.
Examples : The definite article before unique things
Before surnames in the plural to indicate the whole family.
Examples : The definite article before surnames
The Smiths were in New Zealand last month.
The Johnsons have a new car.
The Clintons are guilty for many wars.
Before names of the rivers, seas, oceans and chains of mountains.
|The South Sea
|The Indian (Ocean)
|The Pacific (Ocean)
|The Australian Alps