(1)I forgot_posting__(post)the letter and thought I had left it at home.
(2)Please post the letter for me.I forgot__to post_(post) it this moring.
forget to do 忘记要去做某事.(未做)
forget doing 忘记做过某事.(已做)
The light in the office is stil on.He forgot to turn it off.
办公室的灯还在亮着,它忘记关了.(没有做关灯的动作)
He forgot turning the light off.
他忘记他已经关了灯了.( 已做过关灯的动作)
Don't forget to come tomorrow.
别忘了明天来.(to come动作未做)