Don't forget_____the light when you leave the classroom.

D
forget to do忘记去做某事
forget doing忘记做过某事
turn off关(灯)
turn on开(灯)

D

忘记未做过的事 forget to do

忘记做过了的事 forget doing

D
因为forget to do表示忘记将要做还没做的事，而forget doing表示忘记已经做过的事；turn on指打开，turn off是关掉．