Dear Professor: There are something I must say sorry to you. I handed my classmate's paper to you 'cause I got my paper lost carelessly.I feel very very remorse and uneasy.I blamed for myself and I beg your forgiveness.I promise,I will be responsible and be more careful,not to make such foolish mistakes anymore. yours sincerely, XX 加上点自己的深刻反省吧,诚实还是很重要的啊. 祝你同学好运了.