I won't lend you the book before I have finished it.为什么用have finished

问题描述:

I won't lend you the book before I have finished it.
为什么用have finished

因为是完成时

用have
表示你【完成】的这个动作发生在【借出】的这个动作之前
强调时间的先后顺序

有 before 或 after 的句子,用完成时态或一般时态都可,无太大区别.这句用
before I finish it
也对.

前面有before