There are as you say many cases when to use "the" and it is probably best to learn them a few at a time. Basic rules The first time you talk about something use "a", after that use "the" ........ I have a black car and a blue car. The blue car is faster but the black car is more comfortable. If there is only one of something ............60% of the earth is covered in water Superlatives remember "Simply the best" If the person you are talking to knows what you are talking about. "I'm going to the shop to buy a newspaper" Probably between friends or family and it's the shop you always go to. For exercises etc try to get a copy of Murphy - English Grammar in use

1) When we want to pick out the noun from the group of similar ones or we speak of known things the door of the room the students in our class on the other side of the road the flowers are in the vase 2) When we point to the noun as a standard representer of its class the wolf is a predator the pen is mightier than the sword Popov invented the radio 3) a) When we refer to unique objects the sun the air the universe the world the earth the sky the Bible the Pope b) Rivers, archipelagos, mountains, regions , specified countries the Thames the Alps the Argentine the Oxford Road c) Geographic multi-word names the Black Sea the United States of America d) natural phenomenons the spring the day the night the wind the clouds 4) а) nouns which denotes time at the moment Could you tell me the time, please? in the 2000 5) a) Periodical editions the Times b) before names of disasters we mean at the moment His secretary had gone down with the flu I have the toothache 6) before well-know objects the Coliseum 7) before titles the King the Duke 8) when we have related sentence He is the man I told you about. 9) before the noun, which is defined by suggested group The Oxford of 19 century was unsympathetic the Rome of my youth 10) that, such, proper He is not the person to lay before us the work of absolutely the finest quality This is the place to eat 11) а) before composite proper noun the poet Virgil William the Conqueror the old John Alfred the Great Peter the First 12) a) before nations, tribes the Russians the English the Americans b) before surname to denote the familty the Smiths the Tudors 12) before special adjectives This is the most interesting book I've ever read the second bottle 14) а) in substances of adjectives the good the evil the who the what b) Collective form the poor the dead

Use this article with concrete nouns. And with plural nouns - it is main rule.

the < is use to say something that is not general something specific example if you tell someone. Get me the book. Get me a book. Do you see the difference in the meaning?