Shelley treats the past the way most of us treat someone we have lost. In the first line he does not say that Time long past is *like a memory*. He says it is "like the ghost of a dear friend dead," and that choice sets the terms for the whole poem. A ghost is both absent and present, something that cannot be touched but will not leave. The poem's central idea is that the past works the same way: gone for good, yet able to return with love, sadness and even guilt.
The form reinforces this. There are three stanzas of six lines, and the phrase "Time long past" closes the second and last lines of every stanza, like a bell tolling twice. The rhyme scheme runs ABABBB, and the B sound ("past," "last," "cast") dominates so heavily that it swamps the other rhymes. In the first stanza, "past" itself is the rhyme word three times. Poets usually avoid rhyming a word with itself because it can feel like running out of invention. Here it feels like being stuck, a mind circling the same spot. The short refrain lines, with their three heavy beats ("Is Time long past"), give each stanza a small, flat landing, as if the speaker keeps reaching a conclusion and finding nothing more to say.
Small details of language do a lot of work. In the first stanza the refrain moves from "Is Time long past" to "Was Time long past," and the grammar cannot decide whether the past is still with us or finished. The stanza's list of losses is also odd. A "tone" has "fled" and a "love so sweet it could not last" is gone, which is easy to understand. But "a hope which is now forever past" is stranger, because hope normally faces the future. Here even hope has been filed away as history. The phrase "so sweet it could not last" also hints that the sweetness and the brevity belong together.
The second stanza turns from loss to the muddle of remembering. "Was it sadness or delight," the speaker asks, and never answers. The image that follows, "each day a shadow onward cast," is the most puzzling in the poem. Judging by the wording, I read it as the days throwing a shadow ahead of themselves, so that even in the moment they carried a hint of their own ending. That would explain the wish "it yet might last." We long hardest for a time to stay when we already sense it slipping.
The third stanza is the most painful, and the poem becomes more intimate as it goes. It begins with a friend, passes through shared dreams, and ends with a father beside his child's body. "There is regret, almost remorse" is carefully hedged. The "almost" suggests the speaker is unsure whether guilt is the right word for what he feels. The father watches the "corse" (an old spelling of corpse, which also rhymes with "remorse") "till at last / Beauty is like remembrance." The image is startling because it makes memory a kind of fixing or preserving. The dead face becomes beautiful in the way a memory is beautiful, by being past. The final line, "cast / From Time long past," is hard to untangle. It might mean shaped, like a mould, or thrown off. The syntax strains where the feeling is strongest, and the ambiguity seems fitting.
What makes the poem worth reading is how much it does in so little space. Shelley, one of the major Romantic poets, makes an abstract idea, time, into something with a face and a body, and he lets the form act out the feeling. The refrain returns again and again, the way grief does. The ending is both consoling and unsettling. Loss may be what lets us see beauty at all, since the child's beauty becomes "like remembrance" only once it has gone.