to be wise after the event idiom mainly UK
事后聪明很容易;放马后炮很容易
used to mean that it is easy to understand what you could have done to prevent something bad from happening after it has happened
In retrospect, we should have insisted on checking his calculations, but it's easy to be wise after the event. 回想起来，我们本该坚持检查他的计算结果，不过事后聪明倒是容易。