465. I am writing to extend my sincere gratitude for...
谨以此信表达我对...的真诚感谢。

六级词汇讲解:
本句型是典型的感谢信的开头。其中，sincere意为“真诚的；诚挚的”。如：
Please give my sincere regards to all the members of your family.
请向您全家转达我真挚的问候。

gratitude意为“感谢的心情”。如：
We owe you a debt of gratitude for your help.
承蒙帮助，感激不尽。

本句型中，介词for后面应该接名词或者名词性短语。如：your recommendations（您的推荐）、your attendance in the conference Monday（您出席周一的会议）等。

句型拓展:
I am writing to express my thanks for your presence on my birthday party.
谨以此信表达我对您参加我生日聚会的感谢。
I am writing to show my sincere appreciation for your presence on my birthday party.
谨以此信表达我对您参加我生日聚会的真诚感谢。
I feel deeply indebted to you and I really don't know how to thank you enough for your help.
我感觉欠您的太多，真不知道怎样才能感谢您给予的帮助。