"She is a priest, so she works in the church."
Translation:Hun er prest, så hun jobber i kirken.
This really isn't a subordinate clause. It's a coordinate clause (each can stand independent of the other). I'm assuming that a coordinate clause takes normal word order. A subordinate clause (which is dependent upon the first clause) calls for the verb to be inverted into the second slot. Yikes, I haven't thought this analytically about grammar in years! Enjoying this course so much. Thanks, Duolingo folks.
There are no strict rules unfortunately (I have the same problem). What's more (and ridiculous imo) different Norwegian cities use either of the two and that is specific to each city. I would say that på refers more to abstract objects and i to non-abstract but there are many exceptions. But that's the only rule I could make.