由if引导的条件从句（conditional sentences）可以表达虚拟语气，最常见有2种。
1、表达主观意向与现在实际情况相违背：If+过去式，would+V原型
eg: If I were you, I would choose computer science as my major.
词句亦可以用倒装：Were I you, I would choose computer science as my major.
eg: If I knew him, I would say hello to him.
2、表达主观意向与过去实际情况相违背：If+过去完成时，would+have+过去分词
eg: If I had known him, I would have been in love with him.
此句亦可以用倒装：Had I known him, I would have been in love with him.
虚拟语气先讲到这里，我是完全凭借自己的记忆和经验讲的，不参考任何教辅。以后会陆续给同学们讲解各种从句，让各位在英文写作中将complex grammar运用自如。