What would you call the strip of material that works as a belt for night gown?
And nightgown is one word.
I've never had a nightgown with a tie, but all bathrobes have them.
You will also heard that tie called a "sash."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.