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。