I think I have seen one or two proverbs before that are used in situations where someone thinks a problem/puzzle is very easy but only because he/she has heard the answer to the problem/puzzle.
Is anyone familiar with those proverbs?
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Here's something that might help -
If you say that someone is wise after the event, you mean that they understand a situation and know how to deal with it, but only because it has already happened.
I still feel I should have done more to try and stop him getting into the car, but it's easy to be wise after the event.
I think this seems to be close to what you are asking, alongside "20-20 hindsight" or "hindsight is 20-20" mentioned by @OrbitalAussie.