What is a detailed explanation of Robert Frost's "The Black Cottage"?

"The Black Cottage" by Robert Frost appears upon a first read to be one of his denser and more difficult poems to understand. However, the overarching theme is one easily understood: change, or rather, transience. Through the words of the minister as he speaks to the poet in his company, describing the little black cottage and the woman who lived there for so long, the poem discourses upon the inevitable passage of time and what effect this has upon life, whether as an individual, a member of society at large, or a member of an entire nation.
From the very first line, Frost neatly introduces his subject (the passing of time) via allusion to it, for he (the poet) and the minister (his companion) "chanced in passing by that afternoon" (line 1). This is exactly the sort of thing that time itself is known to do: pass by chance, normally without our conscious awareness until or unless something (a moment of intimacy, or beauty, or violence, or tragedy, for example) makes us aware again that we are transient beings in an ever-changing, ever-evolving world. In the poem, the "something" that captures their attention is "the little cottage we were speaking of, / a front with just a door between two windows, / fresh painted by the shower a velvet black" (lines 5–7) so that "we paused, the minister and I, to look" (line 8).
Beginning in line 14, the minister undertakes his lengthy, detailed, somewhat rambling explanation of the woman who used to live in the cottage, her death, her sons, and her husband, who went off to war and "fell at Gettysburg or Fredericksburg, / I ought to know - it makes a difference which" (lines 31–32). That last bit, about the important distinction between Gettysburg and Fredericksburg, reinforces the poem's theme with irony: to those who fought and died at either Gettysburg or Fredericksburg, as well as to their families, the site would have been tremendously important. But the passage of time dulls even such tremendous tragedies as war and deaths in the memories of those left behind, who were perhaps only indirectly affected in the first place. It would have mattered to the woman in the cottage—mattered enough, clearly, that she kept "a crayon portrait on the wall / done sadly from an old daguerreotype" (lines 23–24) of her husband—but now, the minister (whose job, it might be argued, is to remember all sorts of personal, intimate little details about the lives of the people he has known) cannot recall the details, works his way around the discrepancy, and then dismisses the subject as no longer important enough to consider. The passage of time, indeed, changes everything.
The minister goes on to note how, inevitably, life and time seem to pass by, until the day one looks up and everything has shifted, priorities have reordered themselves, and lives have been lost or born anew. He points out "how forsaken / a little cottage this has always seemed" (lines 34–35) because of "the world's having passed it by— / as we almost got by this afternoon" (lines 43–44). He seems both surprised and unsurprised at how long it has been, perhaps, since he saw the woman alive, since everything he's telling the poet took place and was important to those still living at the time.
The lady herself is described as "innocent" (line 81) in many ways (e.g., her opinions on the Civil War, the values and principles of the nation, and the church and faith, upon which the minister expounds at great length). Of particular interest to Frost's ongoing theme, though, is his reinforcement of the regular and recurring passage of time, whether in decades, centuries, eras, epochs, and how it isn't necessary to "abandon a belief / merely because it ceases to be true" but one can rather "cling to it long enough, and not a doubt / it will turn true again, for so it goes" (lines 105–108). This is a sentiment that echoes the words of the self-styled Teacher in the Book of Ecclesiastes in the Christian Old Testament, who, in his search for meaning, says that "what has been will be again, what has been done will be done again; there is nothing new under the sun" (Ecc. 1:9 NIV).
To conclude the poem, the minister is in the midst of an explanation—what he would believe and do if he were "the monarch of a desert land" (line 112)—when he interrupts his own flow of dialogue by announcing the presence of bees in the wall (line 125) of the cottage, perhaps the one against which the men had been leaning. This effectively ends the conversation, for thereafter "we rose to go" (line 127). This is one more affirmation from Frost that the passage of time, and the oddest moments that interrupt it, is so transient as to be little understood by those of us yet living—in that one instance of distraction, the minister seems to lose all connection to the thread of his impromptu sermon and then does not pick it up again, like so many of the dropped moments and lost opportunities we ourselves face in the river current of time.
https://www.biblegateway.com/passage/?search=Ecclesiastes+1&version=NIV

https://www.poemhunter.com/poem/the-black-cottage/