In a recent column, I explained that past participles are easy. They're all in the dictionary. So if you just know how to find them, you never again have to worry about whether it's "We had sang" or "We had sung." The answer's right there in bold type next to the word "sing." (Spoiler: It's "We had sung.")
But in hindsight I realize that I failed to complete that thought. Thanks to an email I got about the participles column, I now know I should have said: "Participles are easy ... except when they're not."
The "not" part hit me after Sam in Costa Mesa wrote to ask about "hanged" and "hung." This wasn't the first occasion I've had to research the past tense of "hang." But I never bothered to commit the answers to memory. There's no need because help is never farther than the nearest dictionary.