。
She left her umbrella on the bus.
Literal
She [topic-は] bus [on-に] umbrella [object-を] forgot-leaving.
置き忘れる is a compound verb pairing 置く ('place, set down') with 忘れる ('forget') — 'forget by leaving behind,' specifically the absent-minded action of putting something down and walking away. Distinct from a plain 忘れる, which is more general ('forget'). The umbrella-on-the-bus situation is a Japanese commuting cliché — sudden showers leave countless umbrellas in trains, buses, and shops every season, and lost-and-found offices accumulate vast collections of forgotten ビニール傘 (cheap clear plastic umbrellas).