1）时态：可以表示过去，现在和将来的情况。它的基本特点是时态退后。
    a.   同现在事实相反的假设。
    句型 ： 条件从句          主句
    一般过去时      should( would) 动词原形
    If they were here, they would help you.
    
    b.   表示于过去事实相反的假设。
    句型： 条件从句           主句          
    过去完成时       should(would) have 过去分词
    If she had worked harder, she would have succeeded.    
    The rice would not have been burnt if you had been more careful.  
    If my lawyer had been here last Saturday, he would have prevented me from going.
    
    
    If he had come yesterday, I should / would have told him about it.
    含义：He did not come yesterday, so I did not tell him about it.
    If he had not been ill and missed many classes, he would have made greater progress.
    含义： He was ill and missed many lessons, so he did not make greater progress.  
    
    c.   表示对将来的假想
    句型：   条件从句             主句
    一般过去时           should 动词原形
    were 不定式         would 动词原形
    should 动词原形
    If you succeeded, everything would be all right.
    If you should succeed, everything would be all right.
    If you were to succeed, everything would be all right.