water under the bridge: 无法改变的事
He was relieved his time in jail was over and regarded it as water under the bridge.
他为服刑期满而感到如释重负，而且觉得既已无可更改，过去的就让它过去吧。
Don't worry about that mistake, it's water under the bridge.
别担心这个错误, 它已经无关紧要了.

check on: 查对，调查
Can I take a rain check on that?
我可以改日吗？
Will you check on the time the train leaves?
请你核对一下火车开出的时间好 吗 ?

no harm done: 一切平安无事
There, now, you're all right. No harm done.
好了，你现在没事了，没人受伤。
If I sulk for a few days, no harm is done.
如果我有那么几天不高兴, 没有什么妨碍.