十萬火急急急 ar !!!!! 寫信給大學學院 ” 院長 ” 台頭用 english 應該點寫呀 !!! help !

You have to find out his exact title and use it in your letter.
It is not appropriate to call him sir or Mr. xxx, etc.
Your must call him in his title because this letter is a formal letter, especially true in the political and academic fields.
If his is a 院長, you should say :
Dear Mr. Dean of ??? Faculty,
---
I am sorry to be that serious but you cannot make such mistake in this kind of letter. We always address people with his title like below :
Dear Mr. President
Dear Mr. Principal
Dear Master of College

如果你知道佢個姓如姓陳就用--->
Dear Mr. Chan,(女性就用-->Ms. Chan)

如果你唔知道，就用--->
Dear Sir/Madam,

DEAR SIR- -
你寫得比校長都應該係讀過下書架喇WOW= =