how to write letter in English,pls tell me some details

（一）开 头 段：
1、告知对方你的身份（假如对方不认识你）
Dear Sir/Mr. Prometheus,
I was a student at your college, enrolled in Philosophy Department.
/ I am a … at your …
/ I am a … at your college, enrolled in the … course.
/ My name is …, I am ….
2、问候收信人（假如他/她是你的朋友）
Dear Prometheus,
Hello/Hi. How are you? / I hope everything is fine. / How are things going with you?
/ How are you getting on in……?
3、解释写信的原因
（1）致谢：
I deeply appreciate your courtesy and I hope to reciprocate your favor when the opportunity arises.
/ I am greatly indebted to you for (the double) you have gone to on my behalf.
/ Thank you for your letter about studying in Canada.
/ I am writing to you to express my heartfelt gratitude.
/ I am writing to tell you how grateful I am for…….
/ I would like to thank you most sincerely for…….
（2）抱怨：
I am writing to com