Translation:Do you know what the name of Kateřina's girl is?
As a native English speaker I translated this as "Do you know what is the name of Katerina's girl?" Maybe my English is getting rusty from all the time spent on DuoLingo!
''Do you know what Kateřina's girl is called'' should be accepted, I think.
"Do you know what Katerina's girl's name is?" should also be accepted. It is identical in meaning and grammatically correct.
I think that "Do know you what is the name of Kateřina's girlfriend?" should have been accepted. Reported.
That construction is awkward in English. However, "Do you know what the name of Kateřina's girlfriend is?" and ""Do you know the name of Kateřina's girlfriend?" are both accepted answers.
Thank you! I just noticed I did a mistake and I actually wanted to ask about: "Do you know what is the name of Kateřina's girlfriend?" which seems from your post, also wrong.
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.