整个句子的主干是这样子的 Just when(主语）...is exactly（谓语部分） when（表语部分）...,所以主语是Just when it became clear last week that Washington was moving closer to a deal请问when引导从句为何能做主语？这个属于从句里面的活用形式，when引导的从句实质就是一个表示时间的状语，在语法要求不是很严格的情况下，比如在非正式语境下，可以这么用。随便举个例子Where the child was lost was the place where the child was found.