.1 stop doing/to do

stop to do　停止，中斷做某事后去做另一件事。
stop doing　停止做某事。
They stop to smoke a cigarette.　 他們停下來，抽了根煙。
I must stop smoking.　 我必須戒煙了。

典型例題
　 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。由題意可知，她到了山頂，停下來在一個路邊的大石頭上休息。因此，應選擇"stop to do sth. 停下來去做另一件事"。而不僅僅是爬山動作的終止，所以stop doing sth.不正確。

.2 forget doing/to do

　forget to do　忘記要去做某事。　(未做)
　forget doing　忘記做過某事。(已做)
　The light in the office is stil on. He forgot to turn it off.
　辦公室的燈還在亮著，它忘記關了。(沒有做關燈的動作)
　He forgot turning the light off.
　他忘記他已經關了燈了。　 ( 已做過關燈的動作)
　Don’t forget to come tomorrow.
　別忘了明天來。 (to come動作未做)

典型例題
　---- 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 可知燈亮著，即關燈的動作沒有發生，因此用forget to do sth.
而forget doing sth表示燈已經關上了，而自己忘記了這一事實。此處不符合題意。