He forgot _____ a stamp on the l

He forgot _____ a stamp on the letter.
A、put
B、putting
C、to put
D、puts
【正确答案】：C
【题目解析】：forget to do something:忘记要去做某事(这件事情本来要做,还没有做); forget doing something:忘记做过某事(这件事情已经做过,但是忘了)。这句话的意思是：我忘记在信封上贴上邮票了。forget doing sth 与 forget to do sth 是常考的内容点，注意其区别。