Why is it wrong "Kateřina stěžuje si že Matěj neni nikdy doma."
si has to be in the second position. Kateřina si stěžuje...
proc neni at home
"At home" is also accepted in the CZ-to.EN exercise. "At" is not required but can be included.
Why do we need není here? Nidky doma is never at home. If she was complaining that he is always at home would we need je?
Without není there is no "is" in the Czech sentence, as in, "Kateřina complains that Matěj IS never home." And, yes, je would be needed in the Czech translation of "Kateřina complains that Matěj IS always at home."
Thanks, makes sense :)