How we cite our quotes:
It was that night that I found the clue to Ethan Frome, and began to put together this vision of his story. (Prologue.65)
This is the narrator's artistic transformation. The clue he finds is never revealed. Just meeting Zeena and Mattie wouldn't be enough to find the clue, we don't think. Whatever it was, it let him know that Ethan and Mattie were, at least at one time, in love.
"I knew the cat was a smart cat," she said in a high voice, "but I didn't know he was smart enough to pick up the pieces of my pickle-dish and lay 'em edge to edge on the very shelf he knocked 'em off of." (7.123)
The shattering of Zeena's dish seems to spell tragedy and foreshadow the awful events to come. For Zeena, the ruined dish is a symbol of her broken marriage. Our vision of this marriage at the end of the novella is of something broken that has been put back together, and badly at that.
With the sudden perception of the point to which his madness had carried him, the madness fell and he saw his life before him as it was. (8.52)
Ethan thinks his plan to run away with Mattie is madness, not because it's inherently wrong, but because he sees his life as too solid and unbreakable to escape. This is just one of the transformations he will undergo on the day Mattie is supposed to leave.