Is there a rule for using 'i' vs 'på'?
Use 'på' with værelset and badeværelset. Use 'i' for all other rooms, including soveværelset.
This is kinda like the german Word "Die Stube" (Das Wohnzimmer)
When do you use du or i? Are they interchangable?
du is singular, i plural :)
...but make sure it's a capital I, as the lower case 'i' means in.
Can't "stuen" be used simply as "the room" ? Isn't that what it literally means?