1.表達自己對未能按時交論文一事的歉意
2.說明未能按時交論文的原因。
3.表示自己將盡快補交論文并保證以后按時交作業(yè)。
范文：
Dear Mr.Wang,
I am writing you this letter to apologize for my failing to hand in the paper on time.I am very sorry for the inconvenience brought by my delay.Please accept my sincere apology with gratitude.
The reason for my delay was the I was knocked down by a cold.Moreover,I had a fever and was too weak to write a single word.Therefore,it's not in my power to finish the paper on time.I hope you could understand me and excuse me for my delay.
I will do my utmost to finish my paper and hand it in as soon as possible.In my opinion,the importance of punctuality you cannot be emphasized too much,so I assure you that I will hand in my homework on time from now on.
Sincerely yours
Yu Ming
