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