In Polish equivalents of the sentences like: Mark is a student, the subject remains in the basic form (Nominative), but the noun following the verb form of to be, expressing nationality, profession etc. has to be in instrumental case, i.e. Mark jest studentem.

 Instrumental singular - nouns

GENDER	ENDINGS	nominative singular	INSTRUMENTAL SINGULAR
masculine	-em	student (student), Polak (Pole)	studentem, Polakiem
feminine	-ą	studentka (female student), Polka (female Pole)	studentką, Polką
neuter	-em	dziecko (child), miasto (city)	dzieckiem, miastem

after -k- and -g- we have to add "-i-" before the regular ending of instrumental.

Instrumental singular - adjectives

GENDER	ENDINGS	nominative singular	INSTRUMENTAL PLURAL
masculine	-ym, -im	dobry (good), drogi (expensive, dear)	dobrymi, drogimi
feminine	-ą	dobra, droga	dobrą, drogą
neuter	-ym, -im	dobre, drogie	dobrymi, drogimi