。
She left her umbrella behind.
Literal
She [topic-は] umbrella [object-を] absent-mindedly-left.
置き忘れる is a compound verb stitched from 置く ('place, set down') + 忘れる ('forget'), specifically meaning 'to set down and forget, to leave behind by mistake.' Distinct from the simpler 忘れる ('forget'): 置き忘れる puts emphasis on the physical act of setting something down and walking away. Umbrellas are famously among the most-forgotten objects in Japan, with countless 忘れ物 ('lost-and-found items') accumulating at train stations during rainy season — a recurring topic of casual conversation.