Thank-you Letter
词汇：
appreciate感激
convey / express ones appreciation表示感激
cordial衷心的
beyond words难以言表
courtesy礼貌
generosity慷慨
grateful感谢
gratitude感激之情
hospitality好客
moved感动
timely及时的
句型：
1) I take this opportunity to express to you my deep appreciation for the kind assistance you rendered me.
2) I wish there were a better word than thanks to express my appreciation for you generous help.
3) My appreciation to you for your generous help is beyond words. I wish I could repay it one day.
4) Please accept my most cordial thanks for your timely help, which I will always remember.
5) Thank you from the bottom of my heart for your kind help.