Shelley sets two seasons side by side, lets us enjoy the first, and then drops a human being into the second. The first stanza is a June afternoon so complete that nothing in it needs fixing. The second is a winter so hard that even people with every comfort cannot get warm. The final line asks what that winter must mean for someone with no shelter at all. The central idea is that the world's beauty and its cruelty are the same indifferent nature, and that what decides which one you meet is where you happen to stand.
The poem is built from rhymed couplets (*afternoon/June*, *weeds/reeds*, *breeze/trees*, then *die/lie*, *when/men*, *cold/old*) in a loose, mostly iambic pentameter. The effect is calm, almost conversational, like someone telling you what they saw. Shelley then lets his sentences spill across the couplet breaks, so the rhymes never feel like full stops. The winter stanza is one long sentence built from piled-up clauses: "such as when birds die… and the fishes lie… and when, / Among their children, comfortable men / Gather…" Each "and when" adds another weight, until the sentence is cut off by the exclamation of the last line. The two stanzas are also unequal, ten lines for summer and eight for winter. Summer is given more room, and winter is shorter and harder.
The summer stanza is all light and space. The clouds are "floating mountains," a phrase that joins solidity and weightlessness, and the "stainless sky / Opens beyond them like eternity." That simile makes the sky feel endless. Then comes a list: "the weeds, / The river, and the corn-fields, and the reeds." The repeated "and" makes the list feel unhurried, and it includes humble things like weeds and reeds. The verb "rejoiced" is chosen with care, because "All things rejoiced beneath the sun" gives the whole landscape a shared gladness. Even the willow leaves "glanced in the light breeze," which suggests small flashes of light. Nobody is mentioned in this stanza, so nature seems to be enough on its own.
The winter stanza turns that openness into enclosure, and the sounds change with it. Where summer was soft ("silver clouds," "light breeze"), winter is clipped and hard: "Stiffened in the translucent ice," "a wrinkled clod as hard as brick." The detail about the "warm lakes" is especially striking, because it says the cold is strong enough to turn even warm mud to brick. The deaths are seen from a distance. Birds die "in the deep forests," out of sight, while the fishes are visible through "translucent" ice, so the ice is both clear and deadly. Winter here has a terrible beauty, and the poem never tells us how to feel about it.
The turn comes when people arrive. "Comfortable men" is a quietly pointed phrase, and the word has a double edge, since it can mean "at ease" and also "well off." They sit "among their children" and "about great fires," with everything winter should require. Yet they "feel cold." Shelley's argument is an inference: if the protected feel it, the unprotected must suffer far more. He never states this directly. He leaves the gap, and the last line fills it with feeling: "Alas, then, for the homeless beggar old!" It is the only exclamation in the poem and the first time the speaker's own voice breaks through the description. Ending on "old" is also telling, because it rhymes with "cold" and so ties age and cold together at the point of greatest need.
Read this way, the first stanza sets up the second. The weeds and reeds that "rejoiced" in summer are the lowliest things in the landscape, and the beggar is the lowliest person in the human one. Summer includes everyone, and winter picks them out one by one. The poem's quiet force comes from its structure, which moves from sky to ice to hearth to a single old figure. That makes the reader's attention narrow until it rests on the one person the fire does not reach.