The formation of 于是 originates in classical Chinese, where it was not a single word but a prepositional phrase. It literally meant "at this" or "in this (situation/moment)", with 于 meaning "at/in/from" and 是 meaning "this". Through frequent use in narratives and logical explanations, it evolved from simply indicating a location in time or circumstance ("at this point") into a fixed compound that inherently expresses a sequential or causal relationship. Thus, its formation is essentially the fossilization of a spatial-temporal pointer into a connective adverb, metaphorically bridging one event to the next by implying "and then, following from that situation"...