1.‘The' is called the definite article. It indicates a specific one, i.e. a particular one.
The question is easy.
The problems are solved.
The responsibility is yours.
The honesty of the watchman is beyond doubt.
The responsibility to make the meeting a success, is yours.
‘The' is used both with countable (question, problems etc.) as well as uncountable (honesty, responsibility etc.) nouns and both in singular (question) as well as plural (problems) numbers.
2. ‘The' is used before a noun that has been mentioned earlier:
There was a boy. The boy was very intelligent
In the first sentence, ‘a' has been used as we have mentioned the boy for the first time, while in the second sentence; we are taking about that particular boy, whom we have mentioned earlier.
3. ‘The' is used before a superlative degree of comparison:-
This is the longest question.
Mohan is the best student of our class.
4. ‘The' is used before a proper noun when it is used in the sense of comparison:-
Kalidas is the Shakespeare of India.
5. ‘The' is used before a noun when it is clear from the situation which people or thing we are talking about:-
The question was very easy. (a particular question)
The matter was settled. (A Particular matter)
6. ‘The' is used before a common noun when it represents a whole class:-
The cow is a domestic animal (all cows)
7. ‘The' is used before the heavenly bodies and things which are unique in nature or there is only one of its kind:
The sun is very hot.
The planets revolve round the Sun.
The Sun rises in the east and sets in the west.
8. ‘The' is used before the names of rivers, mountains, seas, oceans etc.
a. Before the names of river:-
The Ganga is a holy river.
b. The Pacific Ocean is the largest ocean in the world,
The Himalayas are the highest mountain ranges,
c. Before the names of holy books:-
The Ramayana, The Bible
d. Before the names of famous buildings:-
The Taj Mahal, The White House
e. Before the names of newsapers:-
The Times of India, The Tribune
f. Before the names of a title or post:
The President, The Principal
g. Before the names a nation:-
The English, The Dutch
h. Before the names of a community:-
The Hindus, The Muslims
i. Before the names of trains/ships etc:-
The Rajdhani Express, The Kalka Mail
j. Before the names of countries having a common noun:
The United Kingdom, The United States of America
as shown above