Dear XX,
I am writting this letter for thanking you .when i lost my mobile phone,I was in hurry,you find it,and return to me,How kind you are!
You can't think how glad when i got it!I really appreciate everything you've done!
追答中文的话也可以,但英文更具新意哦
那个发错了别介意,对不起
Thank you found my mobile phone