虚拟条件句即条件状语从句中的虚拟语气，主要包括：
1. 三种基本类型：表示对现在的虚拟，从句用一般过去时，主句用“would+v.”；表示对过去的虚拟，从句用过去完成时，主句用“would have+v-ed”；表示对将来的虚拟，从句用“were to+v.”或者“should+v.”，主句用“would+v.”。比如：
- If I were you, I would study hard.（对现在的虚拟）
- If I had studied hard, I would have passed the exam.（对过去的虚拟）
- If you were to go to the party, you would enjoy yourself.（对将来的虚拟）
值得注意的是，在主语从句中，如果修饰主语的形容词为 essential, imperative, important, insistent, necessary 等时，后面的从句应该用虚拟语气，其形式为“should+动词原形”，而 should 可以省略。比如：
- It is essential that he (should) examine all the facts first.
2. 错综时间条件句：虚拟条件句中的从句与主句所表达的时间不一致。在这种情况下，我们就不可能照搬某种形式，而应该按照句中所表达的时间选择相应的虚拟形式。如：条件句说过去，主句说现在。例如：
- If I had studied hard, I would pass the exam now.
3. 含蓄条件句：句子中没有条件状语从句，但是隐含了虚拟条件句的含义，因此主句要使用相应类别的虚拟语气。比如：
- Without/But for your help, I would not have succeeded.
- You can borrow my notes provided you take care of them
4. 虚拟条件句的倒装：条件句中去掉if,则条件句部分倒装。例如：
- Were I you, I would study hard.
