"It is a menu."
Translation:Det är en meny.
I think it's like... When you say "Det" in the sentence, you are saying a general "it", you haven't identified what "it" is yet, so at that point in the sentence you can't say "den" yet because you don't know that it's a menu. If you were to continue the conversation and say "It is a menu. It is brown" then you could use "den" in the second sentence because you have already identified the menu in the context of the conversation. That is how I would rationalize it, if that makes sense?