?【1】What I have said is wrong.备注(1)：what为融合型关系代词(引导名词型关系分句)：即“集先行词the that和关系代词that为一体”，也就是what=the thing that备注(2):what I have said=The thing that I have said实际上为一个名词短语，名词+后置定语备注(3):名词短语无法表达完整的具有“主语+谓语”结构的事件意义，所以不能使用外置主语。备注(4):以上一般疑问句只能是：Is what I have said wrong?【2】Where Covid-19 virus comes from is not clear.备注(1)：where为疑问副词(引导名词性疑问分句):并在主语从句作介词from的宾语。备注(2):疑问分句能表达完整的具有“主语+谓语”结构的事件意义，所以能使用外置主语。备注(3):Where Covid-19 virus comes from is not clear=It is not clear where Covid-19 virus comes from .备注(4):一般疑问句可以使用外置主语结构：Is it not?clear where Covid-19 virus comes from?总结(1):what,where,when为融合型关系词引导名词性关系分句，不可以使用外置主语结构。总结(2):what,where,when为疑问词引导名词性疑问分句，可以使用外置主语结构。 例句