It is可用的三类词that(should)dosuggested,ordered,required,demanded,requested,proposed,desired等important,necessary,natural,strange等a pity,a shame等as if/as though时间谓语动词的虚拟形式与过去相反had过去分词与现在相反一般过去时(be动词用were)与将来相反would/could/might动词原形与现在事实相反过去时与过去事实相反过去完成时与将来事实相反would/could do
