"That is a menu."
Translation:Det er en meny.
"Det/Dette" is also used in cases where the noun has yet to be introduced; when the gender is still unknown to the reader/listener. In these cases it's functioning as a demonstrative rather than a pronoun.
Once the noun is introduced, and "det/den" is functioning as pronoun replacing the already mentioned noun, it is inflected for gender:
"Det er en meny. Den er ny."
"That (=the thing I'm pointing at) is a menu. It (=the menu) is new."
All Norwegian nouns have a grammatical gender, and are declined accordingly.
en = indefinite article for masculine nouns.
ei = indefinite article for feminine* nouns.
et = indefinite article for neuter nouns.
*feminine nouns may be declined as if they were masculine, so they can take "en" in place of "ei".
Endings are also gender dependent.
The gender a noun is assigned is quite arbitrary, and will have to be memorised with each noun. It does not match up with biological gender, and the only rules that exists are for certain categories of loanwords.