关于虚拟语气的用法，就要考了，急..

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.

主句与从句的动作发生在不同的时间，这时主，从句谓语动词的虚拟语气形式因时间不同而不同，这叫做混合条件句。

If you had asked him yesterday, you would know what to do now.

（从句与过去事实相反，主句与现在事实相反。）

If it had rained last night (过去), it would be very cold today (现在).