When the subject or thing spoken of has already been referred to earlier:
The book you want is out of print. (Which book? The one you want)
When the subject or thing spoken of is:
– A singular noun
– Is meant to represent the whole class
The cow is a useful animal. (OR Cows are useful animals = without articles)
The cat loves comfort. (OR Cats love comfort = without articles)
Before some proper names
The Pacific Ocean, The Ganga, The Suez Canal, The Sahara Desert, The Himalayas, The West Indies, The Unites States, The United Kingdom, The Netherlands, The Ukraine, The Hague
When the subject or thing spoken of is a certain book like:
The Vedas, The Puranas, The Bible, The Ramayana, The Koran,
But we say: Valmiki’s Ramayana (= without articles)
Illiad’s Homer (without articles)
When the subject or thing spoken of is unique of its kind:
The Sun, the moon, the stars, the earth
To make a Common Noun an Abstract Noun:
The warrior (the warlike or martial spirit) in him was thoroughly aroused.
Before a proper noun when it is qualified:
The great Maharana Pratap,
The immortal Kalidas,
The Mr Roy whom you had met last night is my uncle.
With Superlatives:
The darkest cloud has a silver lining.
This is the best book in applied electronics.
With ordinals:
He was the first man to arrive.
The ninth chapter of the book is very interesting.
Before musical instruments:
He can play the flute.
Before an adjective, where the noun is understood:
The poor are always with us.
Before a noun to give the force of a superlative
He is the man (= the chief/concerned/influential man) to talk to.
As an adverb with comparatives:
The more the merrier (è by how much more, by so much merrier)