The idiom 于事无补 is formed by combining two classical Chinese phrases: "于事," meaning "regarding the matter/affair," and "无补," meaning "of no help or remedy." Literally, it reads as "regarding the matter, there is no remedy," structurally following a common classical pattern where "于" introduces the object of concern. Figuratively, it crystallizes into a fixed expression meaning "to be of no avail" or "to not help the situation," conveying that an action is useless in altering an already undesirable outcome. Its formation is a typical example of how classical grammatical structures and concise phrasing evolve into modern idiomatic usage.