Bright Star, would I were stedfast as thou art
Maybe the most amazing thing about this poem is the powerful sense its sounds and rhythms give us of time and the passing of time. The opening of the poem is slow: not only does "Bright Star" have a comma after it that kind of separates it off from what follows, but even the a sound in the word "Star" kind of sounds like the "ahhh" of someone looking up at the heavens, or at fireworks or anything else spectacular. And then the rest of the first line kind of lets that moment resonate, what with the st of "stedfast" alliterating with the st of "Star," and then the vowel of art repeating, but in a little quieter, more closed-up way, "Star's" vowel.
The next line slows us down even further, with long vowels like the o of "one" and the long first syllable of "splendor." But there is a sense of awe and amazement continuing here, too, especially once we hear "night," which rhymes with the "Bright" and kind of fuses the two ideas together in our minds, so that we can visualize the darkness of the night-sky lit-up with the brightness of stars. And the same mixture of darkness and subtle brightness and slowness continues over the next few lines.
Then something shifts in line 5, where we get a new vowel: the o of "moving." You can tell this is an important moment (a) because this vowel hasn't appeared at all in the poem up to this point, and (b) because it now immediately starts repeating itself, in the words "pure," "ablution," and "human" from line 6; "new" from line 7; and "moors" from line 8. The atmosphere of lines 5-8 is chilly and barren; it doesn't seem like a coincidence that the vowel Keats chooses to repeat in these words is the same one from the word "cool."
In lines 9-10 the mood undergoes a shift, however. Now, the lines start picking up the pace, and also increasing in volume and intensity. You can hear this most clearly in line 10, where you get kind of a crescendo effect up to the word "ripening," making the line sound like this: "PILLow'd upON my FAIR LOVE'S RIPEening BREAST." But that's just line 10 – Keats still has four more to go: what's he going to do? Just keep getting louder and louder? No: just as his girlfriend's chest "swell[s] and fall[s]" as she breathes, this crescendo is followed by a return to softness, in the smooth s and f sounds of line 12. To us, these create an atmosphere of profound softness and gentleness that perfectly suits the scene he is describing of the two young lovers lying in bed together. The last three lines of the poem then are somewhere between quietness and urgency. This is especially true in the last line, where you have that extreme dramatic pause after "And so live ever" before getting to the last phrase, "or else swoon to death," which picks up that same melancholy oo sound from lines 5-8 – a sound we haven't heard since then.