、。
She cleaned the room before the guests arrived.
Literal
She [topic-は] guest [subject-が] not-come [during-うちに], room [object-を] cleaned.
~ないうちに is the 'before X happens' construction, but with a built-in implication that the speaker should act *before* the change for some reason — the situation is currently favorable and won't stay that way. Compare: 客が来る前に ('before the guest comes,' simply temporal) versus 客が来ないうちに ('while the guest hasn't come yet,' implying you'd better hurry). The が on 客 marks the subject of the embedded clause 客が来ない. Cleaning before guests arrive is a near-universal courtesy; in Japan it's tied to the broader ethic of presenting one's space respectfully — clean entryway, tidy living room, slippers laid out.