This poem is written in iambic tetrameter: four feet/iambs and a rhyme scheme of ababcc. Written in 1820 or 1821, this was composed in one of the last years of Shelley's life. The poem discusses an evening at Pisa where/when the scene is totally tranquil. One major theme is the dichotomy of change and permanence, perhaps most clearly stated in the line, "You, being changed, will find it then as now."
Shelley uses alliteration to convey the unity of nature and its organized tranquility; just as the different parts of nature are on the same schedule of settling down in silence in evening, the sounds of his words are also in sync:
The sun is set; the swallows are asleep;
The bats are flitting fast in the gray air;
The slow soft toads out of damp corners creep;
In the second stanza, the speaker adds that the air and land are dry. This suggests a distinction, perhaps coincidental or trivial, that there is something different between the stream/river and the land. The one aspect that makes this distinction significant is the description in the third stanza.
Within the surface of the fleeting river
The wrinkled image of the city lay,
Immovably unquiet, and forever
It trembles but it never fades away;
Similar to Keats' "Ode on a Grecian Urn," where the images of people are forever frozen in time, the reflection of the city is permanent on the surface of the water. Because the river is moving ("fleeting"), the image also moves and "trembles." This shows the immortality of the image/reflection/poem as opposed to the movement and mutability of life. The image of the city is "immovably unquiet" - not moving but making some noise; the noise could be the actual noise of the city or more likely it is the sound of the ripples in the water.
The fifth line of the third stanza reads "Go to the . . ." It is considered to be unfinished, with some suggestions as to how the line might have concluded: "Go to the Indies" or "Go to the East." In any case, it is somewhat fitting that there is no specific destination to conclude the line because this lack of a destination could suggest that one could go anywhere and find the same truths: things change but we can find moments and images in nature and poetry in which things are, or seem to be, permanent/immortalized.
In the final stanza, the sun has set behind "cinereous" (ashen, gray) clouds. One light, the sun, is disappearing while another, the evening star (Venus or a bright star in the night sky), appears to take its place. Here again is the dichotomy of change and permanence (together showing the cyclical aspect of nature). One light is leaving (change) but another is arriving - almost as if they've switched places so that one light is always (permanence) "shining through."