Questions & Answers about Podwórko za kamienicą jest ciche rano.
Because the preposition za can require different cases depending on meaning.
Here it means behind in a location sense, so it takes the instrumental case:
- kamienica = base form, nominative
- kamienicą = instrumental singular
So:
- za kamienicą = behind the building
If za expresses motion to a place, it often takes the accusative instead, but in this sentence it describes location, not movement.