Kitchin-no mado-o shimeru-no-o wasureta. (I forgot to close the window in the kitchen.)

Situation 1: Mr. and Mrs. Okubo are going out. After leaving the house, Mrs. Okubo realizes that she forgot to shut and lock the window.

妻: あ、いけない!キッチンの窓を閉めるのを忘れた。