Why can't it be "w roku"?
Well, that's "in a/the year", but as the sentence is clumsy anyway (you sometimes can't avoid that as the learners have limited grammar knowledge at this moment), we're gonna accept it.
Why not "Styczeń to miesiąc roku" ?
That's accepted, it should have worked.
What's wrong with mine? "styczeń jest miesącem z roku"
No "z" is needed. It would mean: January is a month from the year.
thx ^^ for the correction, even though it says roku is in Local case, isn't it genitive then?
In this case it's genitive.
Why not Styczeń to miesiąc z roku?