give me detail explanation of this ode

The poem opens with three consecutive metaphors: the implied, rather than directly stated, comparisons between the urn the speaker is viewing and, respectively, a “bride of quietness,” a “foster-child of silence and slow time,” and a “Sylvan historian.” Of these, the last is perhaps easiest for the reader to immediately comprehend. Ancient Grecian urns were commonly illustrated with scenes or subjects that varied depending on the era and style in which a given urn was created. While more ancient vessels featured paintings of war and heroic deeds, the one Keats had in mind probably came from the early free-style period. Urns of this era are characterized by scenes from religious and musical ceremonies similar to the ones described throughout “Ode on a Grecian Urn.” Because of its subject matter, Keats’s urn must date to before the fourth century B.C., yet the bucolic scenes it depicts have been preserved through the millennia. For this reason, the urn reveals to the viewer a “leaf-fring’d” bit of history: it is a “Sylvan historian.”

More puzzling to readers are the first two metaphors. Each involves the idea of “quietness” or “silence” because the urn relates its story in pictures rather than words. But why is it a “bride of quietness” and a “foster-child of silence and slow time”? The latter may be because while the urn’s creation was the result of a fertile union between an ancient artist and some experience that informed his work, the same artist is now long-forgotten and the experience long-ended. Thus the urn, his “child,” has fallen into the custody of the ages — ”slow time.” People who look at the urn can imagine but cannot actually hear the musical sounds and the story it depicts. Moreover, while in its own day the urn was used by people in their everyday lives, it has since become an artifact, perhaps in a museum, that viewers inspect reverentially — in “silence.”

The most cryptic meaning in these lines is of the word “still.” Is it an adjective, suggesting the urn is “unmoving,” or an adverb, meaning “not yet” deflowered or “ravished”? A dual intent seems to fit the poem best. While “unmoving” suggests the urn’s static condition as an artifact, “not yet defiled” suggests that its beauty, though still present after thousands of years, will one day be destroyed. This points directly to a major theme of the poem: the painful knowledge that all things must pass, including (and perhaps especially) beauty. Though the urn is ancient and might seem eternal, in fact it remains subject to decay and destruction — subject to time, even if, in the case of an antiquity, it seems to be “slow time.” The urn’s perishability is made apparent by a simple understanding: one of beauty’s qualities is that it is rare. Though many urns were created, only few survive, and while this contributes to the speaker’s conception that the urn is uncommon and therefore more striking, it is also evidence that even ancient relics are not immune to time.