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。