Dear Sir or Madam, —— unknown authority/stranger

Dear Mr./Ms./Prof/Dr (Surname), —— known authority

Dear (name), —— friend/relative

    Re: (position) —— only for application letter

As one of your ­­(identity)  s, I am writing this letter to express my (emotion) —— formal

I am (name) , writing to you for (purpose) —— informal

Question 1&2 —— explanation of the whole event and expression of some emotion

Question 3 —— offer your own solution

    Looking forward to your early reply and thanks for your great effort to deal with it.

Yours sincerely