Syntactic expletive

一些语言里面主语会以独立的名词短语的形式强制出现。如英语，如果将主语省略，句子就不合法。这种主语通常称为形式主语（Dummy subject），属于语法虚词（英语：Syntactic expletive）。 一些语言则允许主语省略。如日语通常都可以省略主语，形成无主语句，省略后依据文脉判断主语，动词或其他句子成分不会因此发生变化，

syntax. Longman. Huddleston, Rodney D. The Sentence in Written English: A Syntactic Study Based on an Analysis of Scientific Texts. Cambridge University Press

80–81. Aarts, Bas. Syntactic Gradience: The Nature of Grammatical Indeterminacy. Oxford UP, 2007. pp. 155–156. Aarts, Bas. Syntactic Gradience: The Nature

and the Development of English Determiners, Information Structure and Syntactic Change in the History of English (New York: Oxford University Press),