Six short lines, and almost nothing happens in them. A man walks, a man looks, and something is missing. Shelley's "Stanza" is built on that single feeling of absence, and what makes it so haunting is how little it explains. The speaker never says what the "something" is that "is not there which was." He knows only that wherever he stands in the year, the world has lost a quality it once had.
The seasonal logic is cleverer than it first appears. The speaker names Autumn and Spring, then Winter and Summer, so all four seasons turn up in six lines. But they fall into a mirror pattern. Autumn follows Summer, and it is Summer's clouds he misses. Spring follows Winter, and it is Winter's frost and snow he misses. Each season is haunted by the one that has just left, and the order, Autumn, Spring, Winter, Summer, folds back on itself like a glance over the shoulder. Nothing in this poem is allowed to be simply present.
The imagery carries the same ache. "Autumn's even" uses an old, clipped word for evening, so we get the dusk of the year and the dusk of the day at once. The "dead leaves pass" like a slow procession, and the verb is gentle, neither "fall" nor "die." Its sound is picked up by "was," the word that ends line 4, as if the past tense were hiding inside the present. Spring's "soft heaven" is the sky at its mildest, yet "heaven" suggests something promised, and "soft" is the exact opposite of the hard "frost" he goes on to long for. Even the gentlest scene feels thin. Notice, too, that the one word of open delight, "wondrous," is attached to snow and frost, things that are gone. Present beauty is described as "soft"; absent beauty is "wondrous."
The sound of the poem works the same way. Most lines run to seven syllables with a falling, trochaic beat, as in "WIN-ter's WON-drous FROST and SNOW," a rhythm that leans forward and then drops. Line 2 is shorter, only five syllables: "While the dead leaves pass." The line itself thins out, like the leaves. The rhyme scheme is ABABCC, but every rhyme is imperfect to the ear. "Even" and "heaven" nearly match, yet one has a long vowel and the other a short. "Pass" and "was" look alike on the page and sound different. "Snow" and "now" end with identical letters but do not rhyme at all when spoken. The poem keeps promising a satisfying click and then withholding it, so the form enacts the feeling: a pattern that ought to complete itself and never quite does.
The grammar adds to this. Two "If" clauses pile up before the main statement arrives in line 4, which makes the experience sound habitual, something that happens whenever he walks or looks. Then the syntax breaks apart. Lines 5 and 6 are a list with no verbs, "Winter's wondrous frost and snow, / Summer's clouds," followed by a question. The word "now" appears only once, in the final line, and it is attached to a question with no answer. "Where are they now?" belongs to an old tradition of asking where lost things have gone, usually applied to people or kingdoms. Here the lost things are weather, which makes the grief at once slighter and stranger.
That strangeness is much of the poem's significance. The turning of the seasons is normally a comfort, since whatever leaves will come back. The speaker is not consoled by return. He is caught in a restlessness where each season, while he is inside it, feels like a lack of the last. The bare title "Stanza" suggests, as an inference, a piece of something larger, and the poem does feel as if it breaks off mid-thought. In six lines Shelley turns the seasons from a promise of renewal into a record of what slips away, and he lets the rhyme, the rhythm and the unanswered question make us feel the gap.