.1 stop doing/to do

stop to do　停止，中断做某事后去做另一件事。
stop doing　停止做某事。
They stop to smoke a cigarette.　 他们停下来，抽了根烟。
I must stop smoking.　 我必须戒烟了。

典型例题
　 She reached the top of the hill and stopped ___　on a big rock by the side of the path.　
A. to have rested　B. resting　 C. to rest　D. rest
答案：C。由题意可知，她到了山顶，停下来在一个路边的大石头上休息。因此，应选择"stop to do sth. 停下来去做另一件事"。而不仅仅是爬山动作的终止，所以stop doing sth.不正确。

.2 forget doing/to do

　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。