would have... could have... should have.
The conditional tense is hardly worth bothering about in many other languages. For example in Dari, the variety of Persian John and I learned in Afghanistan, it doesn't exist in any practical sense. The six other languages that I've pondered at some point (German, Spanish, Norwegian, French, Chinese, and Italian) vary widely in their use of this verb tense. A Swede once told me: "I like English best because it is the most accurate language. It can describe anything." I don't really believe that; each language probably reflects what's important to its original culture in a way that no other language can.
But still, we do "regret" very well in English.
It's bothered me: I should have brought my little camera. I almost always have it with me, documenting even the most absurd things.
Secretly hoping to see the owl again, the next morning I took my camera on my run. No owl, of course, but the sun had just come out and the morning frost melts in less than a minute unless you catch it just right.
The sun lit up the blackberry bushes.
And then the fog, covering the valleys like snow. It just doesn't pay to hang on to regrets, I tell myself. I didn't capture the owl with my camera. And that's okay.