Hello, the following is my English composition according to the requirements of the topic, I hope it will help you:

Dear Li Jing:

I am writing this letter to thank you for your kindness. When I was watching the football match last Friday, you found that I left my MP3 player on the playground. You picked up my MP3 player and gave it back to me. I appreciate your great virtue very much.

Best regards,

Li yi

January 2 1 day, 20 12