The first line sets a small puzzle in three words: a "long small room." It can't decide whether to be generous or cramped, and the second line pushes it toward cramped, since the room "narrowed up to the end the fireplace filled." Edward Thomas gives us a space that tapers like a telescope, with the fire at the far end. The speaker's one plain feeling is slipped in almost as an aside: "I liked it." Then comes a shrug about the builders, "No one guessed / What need or accident made them so build." The slightly odd word order, "so build," gives the line an old, unbothered tone, and the reason for the room's shape is lost. Nobody knows why it is as it is.
The second stanza fills the room with watchers. The "moon, the mouse and the sparrow" are all small, and Thomas's verb for them is "peeped," which makes them furtive, like children at a keyhole. Ivy grows "thick" round the casement, so the house seems half swallowed by the outdoors. These witnesses "shall keep / The tale" for "the old ivy and older brick," and that comparative sets up a ladder of age on which the human story sits at the bottom. The poem never says what the tale is. It tells us only that something happened and that it will stay private. The slightly loose rhyme of "peeped" and "keep" suits the hushed mood.
The third stanza turns the picture on the speaker. "When I look back I am like moon, sparrow and mouse." He demotes himself from participant to onlooker, someone who "witnessed what they could never understand / Or alter or prevent." The repeated "or" piles up his powerlessness, and "the dark house" shuts the memory in. The rhyme of "mouse" with "house" is snug and domestic, which makes the helplessness feel small and close rather than grand.
Then comes the sharpest move. The sentence runs across the stanza break: "One thing remains the same--this my right hand // Crawling crab-like over the clean white page." Thomas sees his own writing hand as a separate creature, another small animal in the poem. A crab moves sideways, armoured and scuttling, with no dignity at all. The "clean white page" is a blankness set against the crowded, unreadable past. The hand rests "each morning on the pillow" and then starts "once more," so we picture a daily routine, perhaps writing in bed. Notice the paradox. What stays the same is the hand's crawling, and that crawling carries him "on towards age." The only constant is the thing that moves him toward change. Judging by the poem, the hand may also be what breaks the silence the moon, mouse and sparrow keep, since it is the one witness that can write things down. The poem still never tells us the tale.
The last line doesn't comment. "The hundred last leaves stream upon the willow." After all the looking back and the close-up of the hand, the eye lifts to the window, and the willow from the first line returns. "Hundred" is a number you could almost count, and "last" tells us the count is running out. "Stream" is a clever verb for willow leaves, because the branches hang long and loose, and it also suggests time flowing away. The rhyme of "pillow" with "willow" links the bed and the tree, the human and the natural, so the poem ends on an image of autumn and approaching winter, with no explanation added.
Much of the poem's power comes from restraint. The diction is plain and mostly one-syllable, the rhyme scheme is a steady ABAB, and the rhythm is a loose, speaking pentameter that stumbles and recovers as ordinary talk does. Nothing is declared, and the feeling comes from the gaps: the unexplained builders, the untold tale, and the speaker's quiet admission that he could do nothing. The poem suggests that memory is like that long small room. We can look into it, but we can't change what happened there. Time moves on whether or not we understand the past, and all we can do is keep writing, one page at a time, with the last leaves falling outside.