if + 主语 + 动词过去式
（be 动词一律用 were )
主语 + would ( 或 should )
|与过去事实相反||if + 主语 + 动词过去完成式||
主语 + would (或 should )
+ have + 过去分词
B：谓语用 were to + 动词原形
C：谓语用 should + 动词原形
A：主语 + would (或 should )
注意：（1）、主句中的 should 通常用于第一人称， would 可用于任何人称，同时也可根据意思用情态动词 could, might.
（2）、“与将来事实相反”中的 B 式与 C 式之间的差别是“ were to + 动词原形”比“ should + 动词原形”所表示的
（3）、在与将来事实相反的句中，只有条件状语从句用 should 时主句才能用 B 式的祈使句。
If I were you , I should study English.
If he had time , he would attend the meeting.
If you had come yesterday , you would have met him.
If you came tomorrow , we would have the meeting.
If it were to\should rain tomorrow , the meeting would be put off.
If it should rain , don't expect me.(可接祈使句）
当从句中有 were ,had ,should ,would 等放在主语前可构成疑问句的词时，可省略 if ，用倒装形式。 例如：
Had she more money , she would dress fashionably.
Had you been here earlier , you would have seen him.
Should there be a meeting tomorrow , I would come.
Were I in school again , I would work harder.