Well, the short answer is that "denne" is used with nouns that are common gender (den, en) and "dette" is used with nouns that are neuter gender (den, et) (In the plural it's "disse" regardless of gender) Now, figuring out which nouns belong to which category is the real challenge. There really aren't any sure way to tell, it has to be learned by heart. ................... Further, "dette" (as well as "det") is used in a more abstract sense, when not referring to any particular noun: Hvad er det her/dette (What is this) ("dette" sounds more formal in this example) ............ And then a small correction, hope you don't mind :-) Though your sentences might be acceptable in a very informal context (and then only when spoken), the correct version would be: Hvilken farve er denne/den her bil? Hvilken slags mad er dette/det (her)?