[susan53]

At the beginning of a letter, the salutation may be formal or informal. If informal, then Dear is followed by the first name - eg "Dear Samuel," whereas if it's formal then you use the title and the surname - eg "Dear Mr Johnson," Dear Ms Evans", "Dear Dr Roberts".

English differs from some other languages in that you never....
a) use the first name between the title and surname - eg * Dr Mr Samuel Johnson
b) use the title with the first name only - eg *Dear Mr Samuel
c) use a professional title (with the exception of Dr) - eg *Dear Engineer Brown or *Dear Architect

There's nothing here to really "understand" - it's just convention. Each language has its own conventions for acceptable salutation forms. You just need to find out what they are, and follow them.