Indefinite articles are used with things and people that are not clearly defined.
A man crosses the street. <=> The man (with the hat) crosses the street.
It is clear and obvious that in the first sentence any man crosses the street. A guy that is not defined as someone certain or special. The second sentence defines the man as someone specific that distinguishs himself as the one with the hat.
The use is mostly the same as in English. Therefore just trust your intuition for the use of indefinite articles and you will be fine.