I spent two days in finishing reading this book.为什么不是spent.on?

题目

I spent two days in finishing reading this book.为什么不是spent.on?

答案

spend.on sth.
spend.(in) doing sth.
这两个句子可以互换的.你说的的句子可以改写为
I spent two days on reading this book.因为on是介词,所以动词要加ing.