"whose", how does this fit in, its czyja.
Depending on the (gramatical) gender of the thing in your question it'll be
czyj, czyja and czyje for masc, fem, and neuter respectively.
e.g.
Czyj jest ten stół (masc.)?
Whose is this table?
Czyja jest ta książka (fem.)?
Whose is this book?
Czyje jest to pióro (neuter)?
Whose is this pen?
... and for plural it'll be czyi and czyje for masculine personal and the other respectively:
Czyi to sa synowie (masc. pers)?
Whose sons are they?
Czyje to są zabawki?
Whose are these toys?
BTW. If you don't know which form to use you can always use 'kogo?' instead of every of them, but remember that this will sound a little colloquial.
Cinek