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。