American Expression
In America, we often use the expression

“hindsight’s 20/20.” Hindsight is looking back and 20/20 is perfect vision. Therefore, this phrase means that looking back, we can see clearly. We typically use this expression when we did something that didn’t work out and we feel like we should have known better, but looking back you can always see mistakes that you didn’t as you were making them. This phrase it typically used in more casual conversations, but it’s not necessarily rude to say it in formal situations.
Example Sentences:
“I wish I wouldn’t have made that mistake, but hindsight’s 20/20.”

“You should’ve have answered A!” “Yeah, well hindsight’s 20/20”

“I shouldn’t have done that, but I guess hindsight’s 20/20.”

Hope that this helps :)