Most poems about autumn look out of the window. Longfellow's twelve lines turn the window into a mirror. The speaker announces that it is autumn, then immediately corrects any misunderstanding: the cold is "not without / But within." Outside, spring is under way. What has changed is the speaker, who says plainly, "It is I that have grown old." The poem's central idea is the painful mismatch between a person's inner season and the season the world is living through, a mismatch that makes loneliness sharper.
The first stanza hides a small trick in the word "without." At the end of line one it means "outside," but because the line stops there, we may briefly hear "lacking," as in "not without cold." Two lines later the poem says "within," and the sense settles. The word returns in the second stanza, where the birds are "building without rest," and now it means "lacking" for certain. The first "without" locates the speaker's problem, and the second names what the speaker has lost, which is the restless energy the birds still have. The speaker's own sentence is blunt and nearly all one-syllable words, and the stressed "I" of "It is I that have grown old" works like a confession.
The second stanza is built from motion. Birds are "darting," "Singing, building"; life is "stirring everywhere." These present participles keep the action going and make spring seem endless. Then the stanza ends on the one emotional word in the poem, "lonely," and the word "Save" cuts the speaker out of the general rule. Everything stirs except "my lonely breast." The speaker's isolation comes from the contrast with all that activity, not from any lack of company.
The third stanza changes the kind of image. Spring gives way to the vocabulary of the harvest: dead leaves, sheaves, a flail, a mill. The poem seems to move indoors, to the speaker's inner landscape, where the season really is autumn. Sound drains away through the stanza. "Fall and rustle and are still" is a line that fades as it goes, its linked verbs ending on the quiet word "still." Then come two inverted sentences, "Beats no flail upon the sheaves, / Comes no murmur from the mill." Putting the verb first makes us wait for the subject, and when it arrives it is a negation. The flail and the mill are signs of work and community, so their silence suggests, judging by the imagery, that the busy part of a life has gone quiet. The speaker never says what has caused this, and the poem is stronger for leaving it open.
The form is plain, and the plainness suits the subject. Each stanza rhymes ABAB (without/about, cold/old; air/everywhere, rest/breast; leaves/sheaves, still/mill), which gives a steady, song-like regularity. The lines are short, mostly seven syllables in a falling, trochaic rhythm that ends on a stressed syllable. That stressed ending gives each line a firm, closing thud, so "cold," "old," "rest" and "still" land like small doors shutting. There is no final consoling turn, no promise that spring will reach the speaker too. The last image is a mill that makes no sound.
The poem matters because it takes a very old comparison, autumn as old age, and changes how it works. Usually nature's mood mirrors the human one. Here nature refuses to cooperate, and the speaker's grief is made worse by a world that is cheerfully busy. In a few dozen words Longfellow shows how it feels to be out of step with the season, and his clear, almost folk-song language makes that feeling easy to recognise even if you have never felt quite so old.