Translation:Do you not like scary movies?
I don't see what makes this sentence negative.
"Do you find scary movies unpleasant?" seems a more natural translation from Japanese to English.
And I would translate the English sentence into something like "こわいえいがは好きではありませんか"
I think your sentence carries more the idea of "Don't you like scary movies?" Whereas the sentence they use is like asking "Do you hate scary movies?" One is a positive and the other a negative. If I answer "Yes" to your question, I'm saying I like scary movies, but if I answer "Yes" to their question, I'm saying I hate scary movies.
Where did this い や come from? Is it a verb or a particle? Why wasn't it introduced properly when explaining negation?
This is how literally every word/phrase/sentence is introduced in DuoLingo. On the app, at least.
Update: the suggestion now is "don't like". But isn't いや stronger than that?
movie and movies are just the same in japanese, 映画、why do we have to answer "movies" here?
Because we are talking about a class of movies, which often involves more than one. "Do you not like scary movie" would be incorrect without adding something like "the" before scary.
It accepted my answer of: "Do you like scary movies?" But isn't that answer incorrect?
They're to line up the kanji with the matching kana.
Makes more sense like this:
こわ えいが いや
怖 い映 画は嫌 ですか
Though, Duolingo doesn't accept any of the kanji