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