Any way, i hope that you really walk your talk. otherwise, i will feel guilty for helping you write this letter.
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Dear Mr. Timothy,
Firstly, I am sorry for any inconvenience caused by this letter. I know that you maybe feel hopeless on my performance in your English course. I am also very disappointed by myself. However, I still wish you can give me an opportunity to say something.
首先，我對這封信給妳帶來的不便表示抱歉。我知道，妳可能對我在英語課上的表現已經絕望了，我自己也對自己很失望。但是，我仍然希望妳能給我壹個機會解釋。
As you know, I am a Chinese student. But being different from other Chinese students, I used to be a professional athlete. In China, those teenagers who have sports specialty are sent to dedicated sports school. That is why I have never been to a normal high school before. Therefore, I have not received proper English education before.
妳知道的，我是壹名中國學生。但是和別的中國學生不同，我曾經是壹名專業田徑運動員。在中國，那些有體育特長的青少年們都被送到了專門的體育院校。因此，我從來沒有進過普通高中念書。所以，我也從來沒有接受過正規的英語教育。
Later, I came to Korea for my university. I spent a lot of time studying Korean and my core courses. Therefore, English once again became a serious problem for me. I was very worried about my English. But without a solid foundation, I could hardly make any progress.
後來我到了韓國讀大學。大部分的時間，我都用來學習韓語和我的專業課程了，英語再壹次成為了我壹個嚴重的問題。我對自己的英語十分焦急。可是由於基礎的薄弱，我實在很難有什麽進步。
Mr. Timothy, what I want to say here is that, I am not that kind of student who can easily give up. What I need is some more time. I believe that I can catch up through my effort in future. I surely will spend much more time on English. I walk my talk.
Timonthy先生，我現在想說的是，我不是那種輕言放棄的學生。我需要的，只是多壹點點時間。我相信，通過我的努力，我壹定會把成績趕上來的。我肯定會花更多的時間在英文上，說到做到。
I really appreciate your understanding.
對於妳的理解，我十分感激。
Sincerely,
XXX (妳的名字)