forget to do　忘记要去做某事。　(未做)
　forget doing　忘记做过某事。(已做)
　The light in the office is stil on. He forgot to turn it off.
　办公室的灯还在亮着，它忘记关了。(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)
　Don't forget to come tomorrow.
　别忘了明天来。 (to come动作未做)

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，而自己忘记了这一事实。此处不符合题意。