The definite article the is used:

6.    When we refer to the imaginary geographical lines:

The Equator, The Tropic of Cancer, The Tropic of Capricorn, The Longitude, The Latitude

7.    Before the names of some countries or organizations when the name clearly states that it is a union or organization of more than one state or body:

The United States of America
The United Kingdom
The United Nations Organization
The Security Council
The UNICEF

Note that we do not normally use the before the names of other countries. For example, we say France, not the France. Similarly, we say Italy, not the Italy.

8.    Before unique common nouns:

The sun, the moon, the earth, the sky, the ocean

9.    Before superlatives:

John is the best student in my class. (NOT John is best student in my class.)
She is the tallest of the three sisters. (NOT She is tallest of the three sisters.)
He is the oldest man in our neighbourhood. (NOT He is oldest man in our neighbourhood.)
The cheetah is the fastest animal on earth. (NOT The cheetah is fastest animal on earth.)

10.    Before an adjective which represents a class of people or things:

The rich know nothing about the problems of the poor. (Here the rich means all the rich people and the poor means all the poor people.)
He is collecting money for the blind. (Here the blind means all the blind people.)

Note that the expressions the blind, the dead, the rich, the poor etc., are always plural. They are, therefore, followed by plural verbs.

11.    Before a noun when special emphasis is needed:

He is the man who matters.
Gandhi is the man of the twentieth century.
Now is the time to escape.
This is just the thing I wanted.

12.    Before the comparatives used as a balancing adverb:

The higher we go, the cooler it gets.
The more, the merrier.
The more he reads, the less he understands.

13.    Before an epithet attached to a person’s name:

Alexander the Great
George the Fifth
Edward the Eighth

14.    Before the names of important events:

The Reformation
The Mutiny
The French Revolution
The Second World War