"Why are you sad?"

Translation: Dlaczego jesteś smutna?

[hughcparker]

Why is it smutna, rather than smutnego?

[immery]

if the adjective is "alone" after być/jest ... adjective takes nominative.

[mihxal]

It can be: "Dlaczego jesteś smutna/smutny (when you talk to a woman or to a man respectively)?"

[Jeanne257859]

smutna was not given

[Jellei]

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.

[SteveLamb2]

Dlaczego is used to mean why but when would you,if ever, use Czemu?

[Jellei]

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.

[SteveLamb2]

OK, thanks. One downside of using Memrise alongside Duolingo, I guess.

[RobertGadz]

The answer they give is smutne.

[Jellei]

Given that Polish is way more specific about the word "you" than English is, there are several equally correct (given the lack of context) answers. Putting formal options aside, you have:

"Dlaczego jesteś smutny?" = "Why are you sad, Robert?" (masculine singular)

"Dlaczego jesteś smutna?" = "Why are you sad, Sue?" (feminine singular)

"Dlaczego jesteście smutne?" = "Why are you sad, Sue and Susan?" (plural, women only) -> so that's the option you must have seen

"Dlaczego jesteście smutni?" = "Why are you sad, [Robert and George/Robert and Susan]?" (plural, at least one man)