请分别解释以下、从句、主句在虚拟语气中的时态
　If I had worked hard at school,I would be an engineer,too.
　　如果我在学校学习刻苦的话,我现在也会成为一个工程师了
　　If they had informed us,we would not come here now.
　　如果他们通知过我们的话,我们现在就不会来这里了.
If he were free today,we would have sent him to Beijing.
　　如果他今天有空的话,我们会已经派他去北京了.
　　If he knew her,he would have greeted her.
　　要是他认识她的话,他肯定会去问候她了.