Translation:Do you know what the name of Kateřina's girl is?
This is an informal, "everyday" explanation, but maybe it will help... Think of the difference between "Do you know what it is?" and "Do you know what is it?" The first is correct, the second is not. In this exercise "it" is a phrase -- "the name of Kateřina's girl/girlfriend." So to be correct, the phrase should come before "is," rather than after it.