同学你好，该知识点来自沪江网校《新概念英语2、3、4册连读【双11专享班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
by then是一个固定短语，意思是“到那时”，同学的理解是对的。at that time指的是一个时间点，by then指的是截止到那个时候。
By then, it was October and we were more than halfway through our tour.
那时已是10月，我们已经完成了行程的一大半了。
We all knew by then that the affair was practically over.
到那时，我们都知道事情实际上已经结束了。
It had been supposed that by then Peter would be married.
人们一直认为到那时彼得应该结婚了。

祝同学学习进步~