The Definite Article 'The' is used in following cases
1. While speaking of a particular person or thing or one already referred to.
Example: She found a purse. The purse contained a golden chain. The golden chain is very precious.
2. When a singular noun is meant to represent a whole category class.
Example: The dog is a faithful animal. (Or we can say Dogs are faithful animals.)
3. The is used before superlatives.
Example: She is the most beautiful girl in our college.
4. 'The' is used with the names of renowned buildings, gulf, river, ocean, sea etc.
Example: The Tajmahal, The Persian Gulf, The Char Minar, The Pacific, The Ganga, The Red Sea, The Yamuna etc.
5. 'The' is placed only before the plural names of islands and the mountain ranges, chains of mountains, plural names of countries.
Example: The Netherlands, The Phillipines, The Bahamas, The Himalayas etc.
('The', is not used before the names of countries but if the name of the country contains words like; States, Kingdom, Republic, we use 'the' before them. e.g., The USA,The USSR, The UK, The Republic of Ireland, The Domonican Republic etc.)
6. 'The' is used before names consisting of adjective + noun (provided the adjective is not East/West etc.)
Example: The Arabian gulf, The new forest, The High Street etc.
7. 'The' is also used before names consisting of noun + of + noun.
Example: The Cape of Good Hope, The Boy of Biscay, The Gulf of Mexico etc.
8. 'The' is used before the adjectives East/West etc. + noun in certain names.
Example: The East/West End, The East/West Indies, The North/South Pole.
9. 'The' is also used before the name of direction.
Example: The East, The West, The North, The South.
10. 'The' is used before the name of persons (family) in plural.
Example: The Raymonds, The Ambanis, The Birlas etc.
11. Before the names of important and renowned books.
Example: The Ramayana, The Mahabharata etc.
12. Before such common nouns that are names of things unique of their kind.
Example: The Sun, The Earth, The Sky, The World, The Sea, The environment etc.
(a) The Sky is dark and the Moon is shining.
(b) The Sea seems calm today.
Note: (i) In following cases we do not use 'The' before 'Sea'
(a) We go to sea as sailors.
(b) He is at sea now-a-days. (on a voyage)
(ii) We can use 'the' before 'Space' if it means place but if it means area beyond the Earth's atmosphere, do not use 'the' before it.
(a) He tried to park his car there but the space was too small.
(b) There are lacs of stars in space.
13. Before terms referring Nationality or community.
Example: The Indian, The French, The American, The Hindu etc.
14. Before a proper noun, only when it is qualified by an adjective.
Example: The great Chandragupta, The Immortal Shivaji, The Brave Bhagat Singh etc.
15. With ordinals like
Example: (a) He was the first man to stand up.
(b) The sixth chapter of this book is important.
Note: First, Second, Third......... are called ordinals while One, Two, three etc are called cardinals.
16. Before musical instruments and name of inventions.
Examples: (a) He can play the flute/the tabla/ the harmonium well.
(b) who invented the telephone?
17. Before an adjective, when the noun represents a class of persons.
Examples: (a) The young will support Narendra Modi.
(b) The poor can be trusted.
18. Before a common noun to give it the meaning of an abstract noun.
Examples:
(a) The moralist in Gandhi, revolted against the injustice.
(b) The judge in her prevailed upon the wife.
19. 'The' is used before name of Newspaper, community, political party, historical event, train ship, Aeroplane etc.
20. When two or more nouns refer to one person, put 'the' before the first noun only. If both the nouns refer two different persons or things, 'the' is used before both of them.
Example:
(a) The producer and financier was present there. (producer and financier is one person)
(b) The producer and the financier were present there. (both are different persons. )
21. 'The' is used as an adverb with a comparative.
Examples: (a) The more she gets the more she demands.
(b) The sooner you complete the better it is.
22. 'The' is used before comparative degree being used for selection or comparison.
Example: (a) He is the stronger of the two.
(b) This is the better of the two novels.
23. When a person is being referred by his designation.
Examples: The chairman, the director, the president etc.
24. when the thing referred is understood.
Examples:
(a) Kindly return the book. (That I gave you)
(b) Can you turn off the light.
25. 'the' is used in some phrases also.
Examples:
(a) What is the matter?
(b) Come to the point.
(c) She came to the rescue.
(d) Keep to the left.
(e) The market is hot with the rumor.
(f) He is in the wrong.
26. In phrases; go to the cinema, go to the theatre etc.
Example: She went to the cinema etc.
27. Before names of committee, club, foundation and trust.
Examples: The Lions club, The Rotary club, The united nation, The WHO, The RSS etc.
I hope I have covered all the points. If any point is missing please mention.
You should also see Where to not use 'the' - Definite article.