"잠자기 전에 이를 닦아!"
Translation:Brush your teeth before sleeping!
In English, we say "Brush your teeth before going to sleep". Why you don't count this as correct is beyond me.
before sleeping, brush your teeth AND brush your teeth before going to bed should both be correct
it has to be made into a noun with 기 for it to be grammatically okay before that post-position 전
잠자기 전 like a single noun meaning "before the (act of) sleeping"
잠자다 itself is already a contraction of 잠을 자다,so to be technical:
자다+ㅁ/음 + 를/을 + 자다 + 기 전