請(qǐng)分別解釋以下、從句、主句在虛擬語氣中的時(shí)態(tài)
　If I had worked hard at school,I would be an engineer,too.
　　如果我在學(xué)校學(xué)習(xí)刻苦的話,我現(xiàn)在也會(huì)成為一個(gè)工程師了
　　If they had informed us,we would not come here now.
　　如果他們通知過我們的話,我們現(xiàn)在就不會(huì)來這里了.
If he were free today,we would have sent him to Beijing.
　　如果他今天有空的話,我們會(huì)已經(jīng)派他去北京了.
　　If he knew her,he would have greeted her.
　　要是他認(rèn)識(shí)她的話,他肯定會(huì)去問候她了.