"A door key"

Translation: Klucz do drzwi

[pacecarotepatate]

Which case is the word drzwi here?

[immery]

Genitive/dopełniacz But drzwi for some reason has nominative=genitive=accusative (that is not usual)

[pacecarotepatate]

thank you! have a lingot

[Ash473779]

Is the 'do' needed because 'drzwi' has no recognisable change in it's genitive case form?

[Jellei]

It's needed because it's a key to the door(s), and it doesn't belong to it/them :)