"Co žere mouchy?"

Translation: What eats flies?

[EPTLCowT]

What do flies eat

[endless_sleeper]

I guess you are asking why is that wrong? Perhaps state it more clearly next time ;-)

No, "mouchy" is in accusative here. Also the verb is in singular.

[TQrhe14U]

Thanks! I wondered about that, too. Of course the verb would be plural if the flies were doing the eating. Why was I so slow??

[MezzikTools]

This just doesn't make sence. On this stage of learning, please, make it more specific

[AgnusOinas]

• What eats flies? - Co žere mouchy?
• Spiders eat flies. - Pavouci žerou mouchy.
• What do flies eat? - Co žerou mouchy?
• Flies eat sugar. - Mouchy žerou cukr.

[VladaFu]

What does not make sense? What more specific? Perhaps you want to react to some existing comment on this page?

Please see the answer by endless_sleeper and the Tips and notes about Czech cases and the free word order.
Because the verb is singular "žere", it is coupled with a singular subject. Therefore the subject cannot be the flies. If it were "What do flies eat?" it would have to be "Co žerou mouchy?" with a plural verb.