This sentence uses "i" but my last sentence was "Hun er på badeværelset." I wrote it with "i" instead of "på" and was marked incorrect. Do we use different prepositions for different rooms in Danish?
Yes. It's a little odd now that i think about it, but we do.
"Hun er i stuen" (the living room) "Hun er i køkkenet" (the kitchen) "Hun er i soveværelset" (the bed room) "Hun er i kælderen" (the basement/cellar) "Hun er i gangen" (the hall)
"Hun er på badeværelset" (the bathroom) "Hun er på kontoret" (the office) "Hun er på sit værelse" (her room)
In a few cases it might be correct to use either, but normally you can't. For example you might say "hun er på gangen" (in the hall) but to me that implies a place like a hospital with long halls, not just someone's little hall in their home.