8.2 forget doing/to do
forget to do 忘记要去做某事，forget doing 忘记做过某事。例如：
The light in the office is stil on. He forgot to turn it off. 办公室的灯还在亮着，他忘记关了。（没有做关灯的动作）
He forgot turning the light off. 他忘记他已经关了灯了。（已做过关灯的动作）
典型例题
---- The light in the office is still on. ---- Oh，I forgot___.
A. turning it off B. turn it off C. to turn it off D. having turned it off
