Shelley gives his opening flower just two lines to live. "The flower that smiles to-day / To-morrow dies": the whole life of the thing fits between "to-day" and "To-morrow", and the line break does the killing, with "smiles" in one line and "dies" in the next. That speed sets the argument of "Mutability". Everything we love is temporary, and the loveliest things seem to be the quickest to go.
Each of the three stanzas looks at this from a different angle. The first speaks of the world's delights in general, which "Tempt and then flies", as though they were teasing us. Its central image is lightning, which "mocks the night". A flash of lightning shows how dark the night is, and leaves it darker afterwards. The stanza's last line, "Brief even as bright", links the two adjectives so closely that brightness and brevity start to feel like the same thing.
The form is tightly controlled. Every stanza has seven lines and rhymes ABABCCC: day/stay and dies/flies, then delight/night/bright. The pattern repeats exactly, which gives the poem a settled, almost inevitable feel, and the triple rhyme at the end of each stanza closes like a lid. The lines are short, alternating between three beats and two, so the brief ones ("To-morrow dies", "Tempts and then flies") land like dropped stones. The vocabulary is also strikingly plain, and almost every word is a single syllable. The only grand abstraction is the title, as if the poem were translating a big idea into flowers, lightning and sky.
The second stanza names the human goods: "Virtue, how frail it is! / Friendship how rare! / Love, how it sells poor bliss / For proud despair!" The exclamations sound like sighs. Love is cast as a bad businessman, handing over a little happiness and taking despair in return. "Proud" is a puzzling word here. I read it as despair that holds its head high and refuses comfort, but the poem doesn't spell this out. Then comes the stanza's real shock: "But we, though soon they fall, / Survive their joy, and all / Which ours we call." We might expect the sorrow to be that we lose good things. Shelley's sorrow is harder, because we go on living after they are gone. "Survive" turns endurance into a kind of punishment.
The last stanza is one long sentence built from three "Whilst" clauses: while skies are blue, while flowers are gay, while "eyes that change ere night / Make glad the day". The repeated word keeps us waiting for the main verb, and it also reminds us that these pleasures last only "whilst". The changing eyes are a quiet masterstroke, because mutability now lives in a human face as well as in the weather. Even the hours "creep", a verb that makes calm sound both slow and slightly sly.
When the verb finally arrives it is a command: "Dream thou". It is half permission, half warning. Enjoy the sleep, the poem says, but know how it ends: "and from thy sleep / Then wake to weep." The soft rhyme of creep, sleep and weep sounds like a sigh, and the "Then" before "wake" makes the grief feel not just likely but scheduled. The poem has also moved from "he", "it" and "we" to a direct "thou", so the closing tears belong to the reader as much as to anyone.
What makes the poem significant is that it refuses to console. Many poems about passing time end by finding a comfort, a lesson or a hope, but judging by the text, this one offers none. Its only gesture toward comfort is the dream, and that is presented as the cause of the waking grief. In twenty-one short lines it manages to be musical, simple and bleak at once. The images are ordinary ones, a flower, a flash of light and a blue sky, which is why the ending hurts. Anyone who has watched a good day end can recognise exactly what Shelley is describing.