Translation:Which of your knees is hurting?
Why is ''Which of your knees aches?'' not correct?
Alternatives with forms of "ache" have been added.
Is this translation correct? I don't know how you get 'of your knees' from the Czech. I would translate it as Which knee hurts/is hurting you.
Both are accepted.
My answer should be accepted: Which knee does it hurt you?
The same problem as the last one. There is no place for the "it" in your translation. So no, it should not be accepted.