Burns gives us two winters and lets only one of them end. In the first eight lines the woods that were lately "gladsome green" have been stripped by "winter blasts", yet the mood is almost cheerful, because "maiden May, in rich array" will bring everything back. In the second eight, the speaker turns the same season on himself and finds no May waiting. The central idea is the gap between nature's circle and the human straight line: the year can begin again, a life cannot.
The two stanzas are built as mirror images. Both open with "But", and both rely on the word "shall". In the first it is a promise: May "shall bring them a'". In the second it becomes a refusal: "nae kindly thowe / Shall melt the snaws of Age". The grammar of hope is kept and then turned inside out. There is a smaller shift too, from "our joys" in the first stanza to "my white pow" in the second. The general sorrow of the seasons narrows to one aging head.
The imagery does the same work. Stanza one is crowded with festive, human behaviour: woods "rejoic'd", flowers are "laughing" and "gay" in "double pride", and May is a young woman in fine clothes. Nature here is a party. In stanza two the speaker becomes the landscape, but only the bare version of it. His white hair is the snow, and there is no thaw. "Kindly" is a good word to notice, since in Scots it can mean natural and gentle as well as kind, so the missing warmth is both a fact of nature and a missing mercy. Then comes the strongest picture: "My trunk of eild, but buss or beild", an old tree trunk with no bush and no shelter. The woods that rejoiced in stanza one have shrunk to a single exposed tree, and the verb "sinks" suggests a slow collapse under "Time's wintry rage". Weather is no longer scenery here. It is an attacker.
The sound of the poem is more cheerful than its message, and that is part of its sting. Internal rhymes keep chiming through both stanzas: "seen" and "green", "showers" and "flowers", "May" and "array" in the first, then "pow" and "thowe", "eild" and "beild", "time" and "prime" in the second. In spring the echoes sound like dancing. In winter the same device sounds like a bell tolling the same few notes. The lines alternate between longer and shorter, roughly four beats and then three, which gives a ballad-like lilt, and the title calls the poem a song. The short lines "But now our joys are fled / On winter blasts awa" pass quickly, as if the joy had been blown out of the verse itself.
Burns also knows when to drop the figures. "Oh, Age has weary days, / And nights o' sleepless pain" has no metaphor at all, just a plain account of what old age feels like. After the trees and snow, this blunt statement lands hard. The colours help too. Stanza one is green, stanza two is white, and the one gleam of gold is saved for the thing that is lost: "Thou golden time, o' Youthfu' prime". Youth is the only thing addressed directly in the poem, a rare moment of apostrophe, and the question that closes it, "Why comes thou not again!", gets no reply. Stanza one ended in consolation. Stanza two ends in a cry.
The poem matters because it takes one of the oldest comparisons in poetry, the seasons of the year and the stages of a life, and makes it hurt by refusing the usual comfort. It does this in about a hundred words of Scots and English, with words like "pow", "thowe" and "eild" giving the grief a homely, spoken warmth. Try reading it aloud and you'll hear how the tune stays sweet while the meaning turns bitter, and why that contrast is hard to forget.