Once I wrote "Czyje lustro jest tej", and another time I wrote "Czyje jest tej lustro". But I don't know why they were wrong. Thoughts, anyone?
Are there multiple ways of writing this?
Second possible answer is "Czyje jest to lustro?".
You can't use "tej", lustro is neuter and has to be followed by "to" in correct case. "Tej" is a feminine form of "this" in genitive, dative and locative.
i answered : "Czyje jest to lustro?" and it was written as incorrect
It seems a bit different grammatically, but just a bit, let's disregard it ("Whose is this mirror" doesn't strike as the most probable English sentence). Added now.
Can i say "czyje jest to lustro" too?
Yes, you can.