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
答案：C。由the light is still on 可知灯亮着，即关灯的动作没有发生，因此用forget to do sth.而forget doing sth表示灯已经关上了，而忘记了这一事实。此处不符合题意。