虚拟语气用于条件状语从句中
（1）表示与现在事实相反的假设，条件状语从句中的谓语动词用“过去式（be动词的过去式用were）”，而主句中的谓语动词用“would / should/ could / might + 动词原形”。
如：
If I were a boy, I would join the army.
If the had time, she should go with you.
（2）表示与过去的事实相反，条件状语从句中的谓语动词用过去完成时，主句中的谓语动词则用“would / should / might / could + have +过去分词”。
如：
If he had taken my advice, he would have succeeded in the competition.
（3）表示与将来事实相反，条件状语从句中的谓语动词一般过去时或should(were to) + 动词原形，而主句中的谓语动词则用would / should/could might + 动词原形。
如：
If it were to rain tomorrow, the football match would be put off.
（4）当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时，动词的形式要根据它所表示的时间作相应调整。
如：
If they had worked hard, they would be very tired.(从句说的是过去，主句指的是现在)
以下表格是虚拟语气用于条件状语从句中时，主句和从句谓语动词的形式：
| | 从句(if) | 主句 | | 与现在事实相反 | 主语+did/were | 主语+ would/should/could/might + do | | 与过去事实相反 | 主语+ had done | 主语+ should /would/could/might + have done | | 与将来事实相反 | 主语+ did/ should do/ were to do | 主语+would/should/cold/might + do |
有时侯在使用时可省略if,句子则可换成下列形式，即“were / had / should +主语”。
如：
Were I a boy, I would join the army.
Had he taken my advice, he would have succeeded.
Were it not for the expense, I would go to Britain.
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