Questions & Answers about Konewka stoi za domem obok grabi.
Because after za in a location meaning, Polish uses the instrumental case.
Here, za domem means behind the house as a fixed location, so dom changes to domem.
A very useful contrast is:
- za domem = behind the house / located behind the house
- za dom = to behind the house / movement to that place
With an inanimate masculine noun like dom, the accusative form looks the same as the basic dictionary form, so:
- Konewka stoi za domem. = The watering can is behind the house.
- Idę za dom. = I’m going behind the house.
So the form domem tells you this is about position, not movement.