A substantival adjective is a word that is an adjective but functions as a noun. “Good” is an adjective; but in the sentence below, it is functioning like a noun. We would call “good” in this sentence a “substantive”.
The good was a topic often written about amongst the Greek philosophers.
Greek: Study chapter 9 in BBG. See also predicate adjective and attributive adjective.
Hebrew: Study this document or this video.
