If 条件句中有were, had(助动词)和should时,可使用倒装结构,但if必须省略。
If I had seen him then, I would have been very happy. =Had I see him then, I would have been very happy.
若条件从句为否定句,否定词not应置于主语之后,而不能与were, should, had 等缩成weren’t, shouldn’t, hadn’t而置于句首。
If I were not so busy, I would go with you. =Were I not so busy, I would go with you.