事过境迁 Things have changed
Explanation
指事情过去很久，情况也发生了变化。
Refers to the fact that things have been going on for a long time and the situation has also changed.
Origin Story
小丽和志明是大学同学，毕业后，两人都各自奔赴不同的城市打拼。大学时代，他们一起上课，一起吃饭，一起参加社团活动，感情非常好。毕业五年后，小丽回母校参加校庆，偶然间遇到了志明。两人热情拥抱，寒暄过后，小丽发现志明变了许多。他不再是那个充满理想，意气风发的少年，而是一个在职场上摸爬滚打，略显疲惫的中年男人。他们聊起了过去，聊起了大学时代的点点滴滴，小丽感慨万千。当年青涩的恋情，如今事过境迁，早已成为美好的回忆。两人虽然不再像大学时那样亲密无间，但依然保持着联系，成为彼此生命中重要的朋友。
Xiao Li and Zhi Ming were college classmates. After graduation, they both went to different cities to work. During their college years, they attended classes together, ate together, and participated in club activities together. Their relationship was very good. Five years after graduation, Xiao Li returned to her alma mater to attend a school celebration and happened to meet Zhi Ming again. The two embraced warmly, and after chatting, Xiao Li found that Zhi Ming had changed a lot. He was no longer the young man full of ideals and vigor, but a somewhat tired middle-aged man who had struggled in the workplace. They talked about the past, about the many little things during their college days, and Xiao Li was deeply moved. The youthful love of that time, now that things have changed, has become a beautiful memory. Although the two were no longer as close as they were in college, they still kept in touch and became important friends in each other's lives.
Usage
常用来形容时间流逝，世事变迁，以及由此引起的心情变化。
Often used to describe the passage of time, the changing of things, and the resulting changes in mood.
Examples
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昔日的朋友，如今事过境迁，各奔东西了。
xi ri de pengyou, rujin shiguo jingqian, gebe dongxi le.
The friends of the past, now that things have changed, have all gone their separate ways.
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虽然已经事过境迁，但那段记忆仍然历历在目。
suiran yijing shiguo jingqian, dan na duan jiyi rengran lili zai mu
Although things have changed, that memory is still vivid in my mind.