"It is neither fish nor fowl."
Translation:Det er verken fugl eller fisk.
It is a fixed expression. Switching them around (“Det er verken fisk eller fugl”) is fine, but it is most common to use it as written above.
Tusen takk! I've never heard this expression since I'm not a native English speaker and I'm still very young. :D
It's a spelling variation of "verken".
They both see frequent use, and you can choose to use either, as long as you're consistent about it. "Verken" is the newer (introduced in 1959), simpler version, which is likely to keep gaining ground over the next few decades, so the choice is age rather than dialect dependent.
NB: For listening exercises, only "verken" will be accepted - though the two sound identical. This is just a technical limitation.