、。
When you open the window, please don't close the curtains.
Literal
Window [object-を] open time [topic-は], curtain [topic-は] close-not [polite-request-でください].
Standard polite signage register. ~時は ('when X-ing') marks the condition. Note the two は in the sentence — 時は (temporal topic) and カーテンは (contrastive — don't close the *curtains* specifically). The contrastive は here implies: 'open the window, but as for the curtains, don't close them.' Subtle but natural.