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。