Moonlight on a fountain, a breeze, a hazy mountain half-hidden by trees: the first stanza builds a picture of evening calm, and the second knocks it aside with a single word, "But." Everything lovely in the landscape, the speaker says, is outranked by "thy tone of affection," a voice so gentle it "scarce seemed to break on the stillness of eve." The central idea is that love survives as memory, and that memory, once fixed in the heart, resists the "fast rolling year." The title addresses someone named Harriet, but the square brackets make the name feel supplied from outside the poem. Inside it, we learn nothing of her appearance or circumstances, only her voice, and that choice keeps the poem focused on sound and feeling.
The construction is neat and songlike. There are four quatrains rhymed ABAB, and the lines run on a lilting, galloping beat of roughly four stresses, with extra unstressed syllables that make the rhythm roll rather than march. A pattern runs underneath the rhymes. The first and third lines of every stanza end in a feminine rhyme (fountain/mountain, affection/recollection, sighing/flying, for ever/never), where the rhyme falls on a stressed syllable followed by a soft, unstressed one. The second and fourth lines end firmly on one syllable (breeze/trees, eve/live, ear/dear, year/sincere). The effect is a sigh followed by a settling. The rhyme of "eve" with "live" is imperfect, but it quietly joins the evening setting to the idea of lasting life.
Shelley's language builds on repetition. "Sweet" opens three lines in a row in the first stanza, each pointing at a scene with "yon," and then "sweeter than all" lifts the poem to its real subject. The same trick works with "dear," which keeps returning: "dear recollection," "dear voice," "so dear," "dearest friend," "dearest one." Taken one at a time, these words are stock phrases, and the poem sometimes feels like a writer trying on a ready-made sentimental voice. Read as a whole, though, the repetition acts like an insistence, as if the speaker were pressing the word into the page to keep it from fading.
The imagery is more inventive than the diction. The moonbeam "sleeps" on the fountain, which is a small personification that makes the scene hushed. The trees form "verdant arcades," so the woods become a kind of building, a covered walkway for quiet walking. The poem's best move comes in the third stanza, when the landscape returns as a messenger: the "soft-sighing breeze" of stanza one now carries her voice, with "mild accents of happiness" that "lisp in his ear." Nature, which began as a rival to the beloved's voice, becomes the means of recalling it. Even time gets wings, in the compound "hope-winged moments" flying "athwart him."
The most curious feature is the grammar of the speaker's position. He addresses "thou," yet from the second stanza on he refers to himself as "he": "he hears thy dear voice," "He loves thee." This formality makes the poem sound like a vow or a testimony rather than a private murmur. In the second stanza the text we have also contains a blank in the line "For aye in the heart of thy must live." The missing word presumably names the speaker, so we must supply it ourselves. The poem also calls her "friend," not "beloved," which leaves the relationship a little open.
The last stanza is where tension shows. "Reign" makes love a monarchy, "unalloyed" borrows a word for pure metal, and "never, Oh! never" pushes the promise to the edge of exclamation. A reader may sense that such emphatic vows suggest an awareness of how easily time wears things down, though that is an inference from the tone. The phrase "Though the time it is past!" admits as much in a single breath.
The poem's significance lies less in originality than in what it shows. It is a small, graceful exercise in turning a landscape into a memory and a memory into a promise, and its conventional phrases make a good place to study how rhythm, rhyme and repeated words create feeling. It also gives a plain, readable picture of a poet working inside a sentimental style that he would have to push against to find his own.