**씨 vs. **님

The General rules are:

Rule 1)
name+씨 (ex.철수씨)

,  title+님 (ex.과장님)
If the rule is broken, like 과장씨, it sound likes awkward.

Rule 2)
**씨 can not be used to people who are older than me, while **님 should do

Rule 3)
**님 is a way of expression to people who are generally treated as respectable, like 사장님,사모님, 목사님, 선생님, 교수님, 형님 etc.

So,  it is a bit wierd when it used to titles like 비서 (secretary), 학생..

The point is that there are some titles that really match with 님.


2. the word "님" means "sir" , right? like when you want to be more polite you add "님" at the end of the word you call somebody like : "비서님" is more polite than "비서". isn't it like that ? it doesn't  actually change the meaning, right? then how come "형" means brother but "형님" means brother in law? does the meaning change in this case ?