"Do you not know any good markets?"
Translation:Te nem ismersz jó piacokat?
If there's a noun, which is in plural, don't put the adjective in plural form. When there is not a noun in the sentence, then use the plural form of the adjective.
Edit: I've forgotten to tell you, when the adjective comes after the noun, have to use the plural form of the adjective. Also, if the adjective is the predicate in the sentence, plural form of adjective is used. It's a bit complicated, but by and by you will understand it.