虚拟条件句是英语语法中的一种特殊句型，它用来表达一种假设或与现实不符的情况。在英语中，`if`引导的虚拟条件句主要有以下几种形式：
与现在事实相反
从句：`If + 主语 + 动词过去式（be动词用were）`
主句：`主语 + should/would/might/could + 动词原形`
例如：`If I were you, I would take an umbrella.`
与过去事实相反
从句：`If + 主语 + had + 过去分词`
主句：`主语 + should/would/might/could + have + 过去分词`
例如：`If I had arrived earlier, I would have seen her.`
与将来事实相反
从句：`If + 主语 + were to do/should do/动词过去式（be动词用were）`
主句：`主语 + should/would/might/could + 动词原形`
例如：`If he should come here tomorrow, I would talk to him.`
错综时间条件句
当从句的动作与过去事实相反，而主句的动作与现在事实不符时使用。
例如：`If I had worked hard at school, I would be an engineer now.`
含蓄条件句
有时条件从句不直接表达出来，而是通过其他方式暗示，如使用副词、介词短语等。
例如：`But for his help, we would be working now.`
在虚拟条件句中，动词的时态需要根据所表达的时间进行相应的调整，而且虚拟语气通常用来表达一种与现实不符或不太可能发生的情况。
需要注意的是，在虚拟条件句中，如果主句和从句的谓语动作不是同时发生时，虚拟语气的形式应作相应的调整。
希望这些信息能帮助你理解`if`引导的虚拟条件句。