While it's understandable that some people are frustrated with this segment or unit of the Gk tree, IMO it's useful and worth the effort, because Gk does word building and if one learns each of the words having to do with grammar it will help with word building and etymology. επι is ubiquitous in Gk and the verb φωνέω means to "utter, give a sound," and the noun φωνή means "sound." Put together the word επιφώνημα is related to the verb ἐπιφωνέω, which means "to utter in response," which is what an interjection is! The noun ἐπιφώνησις means "exclamation" (Mandeson dictionary). All of the words have similar etymologies that help with word building. E.g., πρόθεση, "preposition," = προ "before" + θεση = put or place, and prepositions are placed before nouns.