因为独立主格结构中有可能一个非谓语动词是过去分词,谓语动词是过去式,这种情况下怎么区分哪个是主句啊???还有He was listening attentively in class,his eyes fixed on the blackboard.这句话哪个是非谓语动词,哪个是谓语动词,为什么?