Which case is the word drzwi here?
Genitive/dopełniacz But drzwi for some reason has nominative=genitive=accusative (that is not usual)
thank you! have a lingot
Is the 'do' needed because 'drzwi' has no recognisable change in it's genitive case form?
It's needed because it's a key to the door(s), and it doesn't belong to it/them :)