寫英文信的開場白應寫什麼??

What kind of letter you want to write?
Dear xxx, (must)
Hello / How are you ?
please inform more details about !

Depends - if this is a formal letter, you do not start with How are you!

eg if it is a letter for a job - I refer to your advertisement posted in [x] on [date] or if you are replying to a letter - I refer to your letter dated [date] addressed to Mr [x].

If it is friendly or less formal letter you can say.

Dear Peter, hope this letter finds you well. I haven't heard from you since we last met at the [place]. ..........

Dear Peter, how have you been? or hope you are doing well. I have not seen you for a while .............