Er det mange amerikanske biler i Norge? -- This is what is called "dummy subject" in English. In English, you would say "Are THERE...". There is an apple tree in the garden -- Det er et epletre i hagen There is a boy in the garden -- Det er en gutt i hagen As you can see, this "det" has nothing to do with the object described. You CAN'T say "*Den er en gutt i hagen". That makes no sense. In French, you would say "il y a". "Il y a un pommier dans le jardin" "Il y a un garçon dans le jardin" "il y a 356 filles dans le jardin" "Il y a beaucoup d'enfants dans ta voiture" - "*Ils sont beaucoup d'enfants dans ta voiture" (unacceptable phrase) Because you say "il y a", we're talking about a dummy subject. "Den" is never a dummy subject, and thus you would use "det". "Den" needs a referent, much like the French "il" and "elle". And when you need a referent, we're no longer talking about dummy subjects. Let me give you some examples with referents: Le livre... Il... --> Boka... Den... La voiture... Elle ... --> Bilen...Den La maison... Elle ... --> Huset... Det (because "huset" is neuter) Finally, in French you also say "il pleut". - "Det regner", where "il" is a dummy subject. Who's raining? Nobody. You just need a subject, so you use "il". Another example is "Il était une fois..." --> "Det var en gang..." I know there is a wikipedia article on dummy subjects if you want to know more about this.

You use "det" if you refer to a state of being, like... how many kids in a car, or how many american cars in Norway. Veien til Oslo - den veien - den går rett fram But "det er langt til Oslo"