2.if so和if not

so和not可用来替代if后面的从句。

This way you can decide whether the book is likely to interest you，and if so，then you can decide which parts of the book will be most useful and interesting．(人教大纲版高三第206页)

这样，你就可以判断这本书是否可能引起你的兴趣，如果是这样，你就可以判定哪几个部分最有用，最有趣。

Are you busy this afternoon?If not，I wish you would stay with me for a while．

你今天下午忙吗?如果不忙，我希望你跟我呆一会儿。