Once the fire was going , he laid stones on the fire. When hot , he
placed them in an empty oil drum with kau kau(sweet potato), corn and greens.
问:学状语从句的省略时,省略的前提之一就是从句的主语必须和主句主语相一致且有be动词时就可以省略,可本句当中,when后省略了they are, 这里的they是指stones,而其后主句主语为he,两省不相同却用了状语从句的省略,请问这是怎么回事?