How we can say: it is easy to understand what you could have done to prevent something bad from happening after it has happened
Imagine someone asks you a question and you don't know the precise answer and when they say you, you say: "oh, I knew it." for example:
Andy: Do you know the what would be sinA in this triangle?
Kevin: Yes, of course. let me think! Aha. Got it. It would be AC divided by BS (AC/BS).
Andy: No Kevin. You're answer was incorrect. It is vice versa. AC/BC.
Kevin. Oh, I knew it.
Andy: Yea (smilingly) [every question will appear to be easy when you know it's answer. (whereas Andy knows the answer already.)]
But how a native speaker would say it?
I've found a possibly proverb:
- You're wise after the event.
In my opinion it makes sense and it is exactly what I am going to say, but the problem is that based on this link it is mainly BrE and I need to know whether is works in AmE too. If not, then please someone let me know how an American would convey the same thing?