"Does he live in my region?"

Translation: Bor han i min region?

[MarisaDinis]

Could it be: bor han på min region?

[Jnwulff]

No

[MarisaDinis]

And is there a rule that explains why not?

[Jnwulff]

Some words i danish uses "I" like stuen (living room) some uses paa like gården (the farm) some can use both like værelset ( the room)

Region can only use i

There is no rule to follow. You just have to know them all

[MarisaDinis]

Ok, thank you very much! :)