Shelley opens with a simile that sounds lovely and turns out to be quietly brutal: "We are as clouds that veil the midnight moon." Humans are not the moon, the steady light, but the drifting things that cross its face. For three lines the clouds are glorious. They "speed, and gleam, and quiver," "streaking the darkness radiantly," and the stacked verbs give a real sense of flicker and hurry. Then the exclamation fades into a flat, final statement: "Night closes round, and they are lost for ever." The brilliance was genuine, but it was never going to stay.
The second image changes the sense involved from sight to sound. We are "forgotten lyres," instruments made for music but left untended, sounding only when the wind passes through. The strings "give various response to each varying blast," and the near-repetition of "various" and "varying" makes the phrase itself shift as you read it. The detail that matters most is that the wind does the playing. The lyre has no say in what it sounds like. The stanza closes on "mood or modulation", a musical term for changing key, so the metaphor stays inside music right to the end. No second motion brings "one mood or modulation like the last."
Stanza three drops the similes and speaks directly. "We rest.--A dream has power to poison sleep; / We rise.--One wandering thought pollutes the day." The clipped clauses, the dashes and the neat pairing of rest and rise make the lines feel like a sequence of small, quick events. The verbs "poison" and "pollutes" are striking. They suggest a mind so fragile that one dream or one stray thought can contaminate hours of experience. The third line then runs through nearly all of mental life, "feel, conceive or reason, laugh or weep," and gives laughter and tears equal weight, as items on a list. Even the last line of the stanza has a twist. We "embrace fond woe," and since "fond" can mean both loving and foolish, the phrase hints that we sometimes cling to our own sorrow.
The final stanza begins with a startled "It is the same!" Whether the feeling is "joy or sorrow," the "path of its departure still is free." This cuts both ways. Joy will leave, which is sad, but so will grief, which is a consolation, and the poem refuses to say which reading it prefers. The last line is a paradox: "Nought may endure but Mutability." The only lasting thing is the force that guarantees nothing lasts, and the capital M makes it almost a ruler over the whole poem.
The form works against the content. Four quatrains rhyme ABAB, and the lines are largely iambic pentameter, so the pace stays measured and almost stately while the subject is instability. Several rhymes fall on unstressed endings, such as "quiver" with "for ever" and "sorrow" with "morrow," and these give the lines a falling, sighing quality. Linking "quiver" to "for ever" also ties flicker to loss. At the very end, the short word "free" is paired with the long, abstract "Mutability." That word is the longest in the poem and takes the final place, a position that is hard to miss.
The poem's significance lies in how much it fits into sixteen lines. It is a compact statement of a Romantic concern with the unsteady inner life, the way a person is never quite the same from one hour to the next. It is also consistent with its own argument. It does not stay with one image but moves from clouds to lyres to plain statement, so the poem itself changes as you read, much as the mind it describes does. And it avoids easy comfort or despair. A reader can finish it grieved that nothing holds, or relieved that nothing, including pain, has to. Both reactions are supported by the text.