Q: Why do we say “To jest nauczyciel” (“This is a teacher), but “On jest nauczycielem” (“He is a teacher”)? Why do we need two different cases since we use the same verb “jest” and the meaning is almost the same?
A: Those two utterances may look similar at first, but the first one, “To jest nauczyciel” is different because the linking word “to” (“this”) excuses the complex predicate “jest nauczyciel/em” (“is a teacher”) from being linked to the instrumental case, as in “On jest nauczycielem”. Thus, when we begin with “To jest…,” we need to leave the noun in the nominative. Also, the meaning of these two is slightly different: using the instrumental case like in “On jest nauczycielem” implies an air of permanence (e.g. teaching is his trained profession), whereas “To jest nauczyciel” does not bear this emphasis (e.g. he may be working in a teaching position, but temporarily or without training).