hindsight is always twenty-twenty

Hindsight is always twenty-twenty. (Also Hindsight is always 20/20 and Hindsight is 20-20.) — Задним умом каждый крепок; Человек всегда задним умом крепок;
≅ Хорошая мысля приходит опосля (разг.,просторечн.)
It’s easy to complain after the fact.
If you knew then what you know now, your choice would be clear.
It’s easy to make the right decision AFTER things have happened.

A better solution is worked out after the first one is already applied and could not be changed. Have you not heard the lament:"I should have done it the other way!"? Задним умом крепок is when a person regrets what is done, but cannot change anything.
Examples:

A. If I hadn’t sold my ocean-view house last year, I’d be a millionaire today!
B. Yes, hindsight is twenty-twenty!

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There's another interesting Russian saying which has to do with it: "Знал бы прикуп - жил бы в Сочи". Any ideas how to convey this idea in English?

— Milanya
Not in English. In Russian it sounds like:
Знал бы где упасть, соломки бы подстелил.

— Samantha writes:
Some Insight
Just to add, usually people use the idiom as, "Hindsight is always 20/20" ~after~ someone has said "If I had done it this way, things would have been so much better..." or something to that effect.

People rarely refer to their "20/20 hindsight" but rather use it as a way to soothe others. It's more of a "What's done is done." type of thing. :) Samantha

Also
[in hindsight]
[wise after the event]
opp.
[think on one's feet]

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This idiom uses twenty-twenty in the optometrist's sense, that is, "indicating normal vision," and hindsight in the sense of "looking back" or "reconsidering." (Answers.com)