An adjective in predicate position is “the word is good“. An adjective in attributive position is “the good word”. An adjective in the substantival position is functioning as a noun. “The good is something we should pursue.”
An adjective in predicate position is anarthrous. When translating, some form of the verb “to be” will be necessary. The following sentences all mean, “The word is good.” Study chapter 9 in BBG.
ο λογος αγαθος
αγαθος ο λογος
Watch the videos for attributive, predicate, and substantival uses of the adjective. Also this.