"Why are you sad?"
Translation:Dlaczego jesteś smutna?
It can be: "Dlaczego jesteś smutna/smutny (when you talk to a woman or to a man respectively)?"
There are more equally correct answers here, the algorithm doesn't necessarily have to put "smutna" there. It can be "smutny", for example.
Although a moment ago this used to have the Formal You answers as the starred ones as well, so that may have been confusing, true. Fixed.
Currently the starred answers are:
Dlaczego jesteś [smutna/smutny]? (singular, female/male)
Dlaczego jesteście [smutne/smutni]? (plural, no men/at least one man)
One of them has to be possible to be created.
It's perfectly synonymous. However, some natives can consider it a worse option, either treating it as a bit colloquial, or saying that it's a Russicism. Most wouldn't bat an eye, though.