Questions & Answers about У двери высокий порог.
Why is it у двери and not just дверь?
Because у is a preposition meaning by, near, at in this kind of sentence. After у, Russian normally uses the genitive case.
So:
- дверь = the door
- у двери = by the door / at the door
This is a very common pattern in Russian:
- у окна = by the window
- у дома = by the house
- у входа = by the entrance
So у двери is not the subject. It is a location phrase.
Why does дверь become двери?
Because дверь is in the genitive singular after the preposition у.
The noun дверь is feminine and belongs to the soft-sign pattern. Its forms include:
- nominative: дверь
- genitive: двери
- dative: двери
- accusative: дверь
- instrumental: дверью
- prepositional: двери
So here:
- у двери = at/by the door
Even though двери can also be other cases, after у it is understood as genitive.