if引导的虚拟语气主要有以下几种形式：
表示与现在事实相反的情况
从句：If + 主语 + 动词一般过去时 （Be动词用were）
主句：主语 + should/would/might/could + do
例句：
If I were you, I would take an umbrella.
If there were no air, people would die.
表示与过去事实相反的情况
从句：If + 主语 + had + done
主句：主语 + should/would/might/could + have done
例句：
If I had got there earlier, I should/would have met her.
If he had taken my advice, he would not have made such a mistake.
表示与将来事实相反的情况
从句：
① if + 主语 + were to do
② if + 主语 + should + do
③ if + 主语 + 动词一般过去式 （be动词用were）
主句：主语 + should/would/might/could + do
例句：
If he should come here tomorrow, I would talk to him.
If there were a heavy snow next Sunday, we would go skating.
错综条件句
从句的动作与过去事实相反，而主句的动作与正在发生的事实不符。
从句的动作与现在事实相反，而主句的动作与过去事实不符。
例句：
If I had worked hard at school, I would be an engineer now.
If they had informed us, we would not come here now.
省略句
有时，虚拟条件句中，如果主句和从句的谓语动作若不是同时发生时，虚拟语气的形式应作相应的调整。
省略从句：He would have finished it.
省略主句：If I were at home now.
倒装语序
当从句的谓语动词含有were，should，had时，if可以省略，这时从句要用倒装语序，即把were，should，had等词置于句首。
例句：
Should he agree to go there，we would send him there.
Were she here，she would agree with us.
虚拟语气用于表示非真实或假设的情况，常见于表达建议、愿望、猜疑或与实际不符的假设等情境。在使用时，需根据不同的时间背景选择合适的虚拟语气形式。