I shall borrow the book ,if it is yours.用虚拟语气写出与将来相反的虚拟条件句

I should borrow the book if it were to be yours.
I should borrow the book if it should be yours.

不能用I should borrow the book if it were yours，这是对现在的虚拟
请参考虚拟语气谓语变化表
虚拟时间 主句谓语 IF从句谓语
将来 would(should/could)+do were to do/should do
现在 would(should/could)+do were/did
过去 would(should/could)+have done had done

对将来的假设/虚拟，if从句有三种写法，楼主可以任意选择其中一种。
1. I should borrow the book if it were yours.
2. I should borrow the book if it were to be yours.
3. I should borrow the book if it should be yours.

I should borrow the book if it were to be yours.

直接is改为were