Dearxxxxxx,
I am writing to extend my sincere gratitude for xxxxxxxxx (感谢的原因). If it had not been for your assistance in xxxxxxxxxxx (对方给你的具体帮助), I fear that I would have beenxxxxxxxxx(没有对方帮助时的后果).
Every one agrees that it was you whoxxxxxxxxxxxxxx(给出细节).Again, I would like to express my warm thanks to you! Please accept my gratitude. Yours sincerely