“势必”作为副词,与它作为跨层结构相比,最明显的区别在于“势”丧失了作主语的功能。判定“势必”虚化的形式标准只有一个,即上下文中存在取消“势”的主语资格的语言成分。我们将副词“势必”的产生时间界定为魏晋南北朝时期,其成词的句法环境是“势必+谓词性成分”。 “Predominance ” as an adverb, the most obvious difference compared with it as a cross-layer structure is that “potential ” has lost the function of making a subject. There is only one formal standard for judging “imperative ”, that is, there is a linguistic component in the context of disqualifying the subject from “potential ”. We define the production time of adverb “imperative ” as the period of Wei, Jin, Southern and Northern Dynasties. The syntactic environment of the adverb is “imperative + predicate ”.