"Are the workers happy or sad?"
Translation:A munkások boldogok, vagy szomorúak?
You need plural adjectives if the subject is plural, too, and if it's a sentence of the form "[Subjects] are [adjective]", a so-called "copula sentence".
In all other cases, most importantly those with "[adjective] [Subject]" (like "the red car"), the adjectives are inserted in their base form, regardless of case or number of the noun. Let's try it:
- az autó piros - the car is red
- az autók pirosak - the cars are red
- a piros autó - the red car
- a piros autók - the red cars