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(guān)了。(沒有做關(guān)燈的動(dòng)作)
　he forgot turning the light off.
　他忘記他已經(jīng)關(guān)了燈了�！　　� ( 已做過關(guān)燈的動(dòng)作)
　don't forget to come tomorrow.
　別忘了明天來�！　　　　　　　� (to come動(dòng)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(guān)燈的動(dòng)作沒有發(fā)生，因此用forget to do sth.
而forget doing sth表示燈已經(jīng)關(guān)上了，而自己忘記了這一事實(shí)。此處不符合題意。