Write in a formal manner. Date Dear Professor. XXXXX/ Dr ......................, I wish to ask you about ............................ Yours sincerely, Your name and surname.

1) usually it`s "professor + last name" 2) Hello is polite enough, unless you want to make it *very* formal - in that case Good Morning/Day/Evening works best 3) Yours Fathfuly? Yours Truly?

Unless told otherwise by your professor, you should definitely use last name with a title, either "Professor Lastname:" or "Dr. Lastname:" if Dr. is appropriate. No other introductory word is necessary, just place the colon after the last name. In the USA, "Sincerely" is the standard closing for a formal or semi-formal letter. "Best regards," or "With regards," are also fairly standard.