Long ago, there was a man from the State of Chu who loved his sword very much. One day, he was sitting in his boat preparing to cross a river, when he accidentally dropped his sword into the water. He immediately made a notch on the side of the boat at the place where his dear sword fell. When he returned close to shore, he re-entered the water just beneath the notch he made, looking for his sword. Naturally, he wasn’t able to find it.
The sword was already gone because the boat and the river were in motion. This idiom is used to describe a person who sticks to rigid rules without considering a changing environment [or describes an action made pointless by changing circumstances].

从前有一位楚国人, 他非常爱惜他的宝剑。有一天,他正坐在船上准备过河,一不小心他就把宝剑掉进了河里。他马上在宝剑掉落的地方作了记号。当他到达对岸的时候,他沿着记号跳进河里去找他的宝剑。当然,他已经找不到了。