This is the 8761st most frequent Polish word.
rosjanin
A Russian man.
Here, 'Rosjaninem' is used as an instrumental form of the noun to describe someone's nationality.
Ten człowiek jest Rosjaninem.
This man is Russian.
In this sentence, 'Rosjanin' is used in its nominative form, referring to a Russian person.
Czy ten Rosjanin mówi po polsku?
Does this Russian person speak Polish?
'Rosjanina' appears in its genitive form, used after a negative verb to indicate the object.