it is used before a noun..or on a proper name! example.. for a noun: "DOG" you can add "the" before it to emphasize the thing, which is "the Dog". example.. for a proper name: "FEMALE", (above, the dog can be either a boy or a girl, right?) that is what we call Proper name, where you should emphasize the gender, not only that, but also what kind the thing is.. so we can say "The female dog".

It depends, some things we always use "the" to describe a single instance of something, or when there is only one of the instance you could be referring to. If you already defined the article, maybe you said "my dog is well behaved", later you could use "the dog" or "this dog" in the same sentence to refer to the same article, because you have already defined it "my dog". Maybe you know this already. But some things like "the world"; we would always use "the" because there is only one "world", so we normally say "the world". Like "The best person in the world" - there is only one world, and there is only one "best person". Similarly, because there is only one (figurative) "top" of our world, you can use "the" to describe this as well, meaning: "The (one) top of the (one) world".

Hello..."on the top of the world" is correct". The word "the" commonly proceeds the noun...in this case ..."world" it is an extremely high frequency word...in fact it is almost impossible to think of a sentence that doesn't use it...actually there are sooo many uses for this word and rules about using it, it is too much for me to write without some kind of help....so here is a comprehensive useage list...your homework :P http://dictionary.cambridge.org/define.asp?key=82277&dict=CALD Hope this helps! :D