I am coming across sentences where человек is used instead of мужчина, when does this happen and why?
Example:
That man is not a professor he is a doctor
Этот человек не профессор доктор
тот человек не профессор он доктор
I am coming across sentences where человек is used instead of мужчина, when does this happen and why?
Example:
That man is not a professor he is a doctor
Этот человек не профессор доктор
тот человек не профессор он доктор
It's just that Russian happens to be more gender-neutral than English, at least in the case of человек which can mean 'man', 'person' or 'human being' depending on context:
Use мужчина only when you want to emphasise their masculinity or when gender is important:
I think Bolsheviks' idea of everyone's equality has contributed to this effect. Before the revolution of 1917 it was customary to use господин/госпожа (or молодой человек / барышня for younger people) in the context you mentioned:
In those days человек was used to call (and refer to) servants.
Btw, человек has an irregular plural: люди 'people':
Человек is a man/human in general. Мужчина is used when you want to highlight that a person is a male as well. For example - A man is strong. Человек силён. But Мужчина по природе своей - воин. A man is a fighter by nature. (here we specify that he is a male) So is you want to say something in general use человек. When you want to add than the person you are talking about is male then use мужчина.