"Den" is used for masculine/feminine nouns, while "Det" is for neuter nouns. For example;
- -Hvor mye koster smørbrodet? / How much is the sandwich?
- -Det koster 15 kroner. / It costs 15 kroner.
- -Hvor er boka? / Where is the book?
- -Den er på biblioteket / It is in the library.
It depends! If you pronounce it like this (dn), it means "I eat it" just like in English. If you pronounce "den" more clearly, it puts emphasis on "it" making it sound like you are eating it and not something else. Example: No empasis: "Hva gjør du med den? Jeg spiser den." (What do you do with it? I eat it.) Emphasis: "Spiser du denne? Nei, jeg spiser den." (Do you eat this one? No, I eat that one.)
Bottom line is that this pronunciation is totally fine :)
I believe that although not needed in everyday smalltalk, many of these odd sentences will stick to your head (thanks Duolingo). You can recall these sentences later and apply them to your specific needs. It's the grammar that counts, not the content. =) Keep calm and watch out for those child-eating wolves.
In this case, "den" would be replacing a masculine or feminine noun, while "det" would be replacing a neuter noun or referring to something more abstract.
"Det" is also used as a default when the noun's gender is unknown, and as a formal subject in sentences like "Det regner" ("It is raining").
denne means "this" when referring to a grammatically feminine or masculine noun.
dette means "this" when referring to a grammatically neuter noun or a noun you've introduced to the conversation just now.
disse means "these" when referring to a plural noun, regardless of grammatical gender.