"A könnyű székek nagyon hidegek."
Translation:The light chairs are very cold.
Does könnyű have a plural form/is this it's plural form? It's an adjective too, right? Are we using the plural form for hidegek (cold) but not for könnyű (light) intentionally, or is there just some irregularity here in how it works?
The plural form of "könnyű" is "könnyűek".
You don't pluralize adjectives if they are directly in front of a noun. So if the sentence were "The cold chairs are very light", you'd say "A hideg székek nagyon könnyűek." <- the adjective in front of the noun remains singular, while the other one -- which is basically the predicate of the sentence -- has to be plural.