The form of this poem is a Petrarchan sonnet, named after a dead Italian guy (can you guess his name?) who wrote lots of love poems. Petrarchan sonnets—named after the Italian poet Petrarch (pat yourself on the back if you got it right)—are made up of two big parts: the octave (the first eight lines) and the sestet (the last six lines). The rhyme scheme of the octave is almost always the same (ABBAABBA—where the letters correspond to the end rhyme of each line), while the sestet scheme can vary. In this sonnet, it is simple alternating rhymes (CDCDCD).
Every sonnet has a turn—a shift in the poem's thinking, also called a volta—and in Petrarchan sonnets, this turn usually occurs at the end of the octave. That's what we see here. It begins to shift in lines seven and eight when Keats discovers Chapman's translation, but really turns at line nine when Keats tells us how that experience made him feel.
Sonnets written in English are almost always written in iambic pentameter, and that's what we have here. If you didn't click the link to read all about it (shame on you!), iambic pentameter means lines of ten syllables with alternating unstressed and stressed syllables (or, iambs). The pattern sounds like da-DUM da-DUM da-DUM da-DUM da-DUM. For example:
And many goodly states and kingdoms seen. (2)
That meter won't be perfect, though; Keats reserves the right to change it when he wants to create an effect. Most often, this means switching the stress in the first two syllables, which we see in the first and last lines ("MUCH have" and "SI-lence"). Those reverse-iamb pairs are called trochees, with the stress of the pair falling on the first syllable, rather than that last.
Why would Keats mess with his iambs? Well, he throws on the metrical breaks with these trochees to really punch up words he wants to stress. Interestingly enough, they come at the first and the last lines of the poem, where emphasis is naturally going to be anyway. Since they work against the rhythmic grain of the rest of the poem, we as readers can't help but have our ears (even our inner ears, if we're reading silently) perk up and take note.
For the rest of the lines, the poem is actually very consistent in its meter compared to most. That explains Keats' use of abbreviated words to make sure that everything fits into his metrical plan. The only other significant variations occur in lines eleven and twelve. For line eleven to be iambic, we have to force Cortez to be pronounced with a strong stress on the first syllable:
or like stout cortez when with eagle eyes.
(But it's possible that it would have been pronounced that way back in Keats' day in England—in his poem, Lord Byron pronounced Don Juan "Don JOO-on" to make his rhyme scheme work.) Line twelve also has an extra unstressed syllable—on the word "pa-CIF-ic." Extra unstressed syllables weren't really considered a true break with the rhythm when Keats was writing, though.
By and large, Keats is right in lockstop with Petrarch's sonnet plan—and we think that makes a lot of sense. Here he is, celebrating the translation of the classic writings of Homer. So—hey, what better way to celebrate a classic work, and its translation, than by expressing oneself in a classic form, which has also been borrowed from an older, foreign writer. With this poem, then, Keats is propping up three writers in one: Chapman, Homer, and Petrarch.