forget to do忘記要去做某事，forget doing忘記做過某事。例如：

The light in the office is stil on. He forgot to turn it off. 辦公室的燈還在亮著，他忘記關了。（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）

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，而忘記了這一事實。此處不符合題意。