"先生" in Engpsh can be translated as "Mr.". "Mr." is a title used before a surname or full name of a man, whether he is married or not. It originated from the word "master", which was used to address a man of some social status or authority in the 16th century.
Here are some examples:
"Good morning, Mr. Smith." (Engpsh)
"早上好，史密斯先生。" (Chinese Translation)
"Can I help you, Mr. Lee?" (Engpsh)
"我能帮助您吗，李先生？" (Chinese Translation)