6.I'm very sorry.I forgot --------- my homework today.

forget doing sth是事情已经做了,但事后忘了曾经做过.如
Oh,I forgot posting the letter!噢,我忘了信已经寄出去了.（信已寄出,说话人事后忘了）
forget to do sth是事情没做,忘记了.如
Don't forget to buy some salt.别忘了买些盐.（盐还未买来,这是说话人在嘱咐对方）