The Romantics, and Keats in particular, did for the modern ode what Bach did for the fugue or what Bill Gates did for the personal computer. It’s their turf, and you can’t step on it without acknowledging the masters.
But the Romantics didn’t invent the ode. It’s actually an old form going back to Ancient Greece. Keats may have been inspired to write about a Grecian urn because he was using a Greek form.
In Ancient times, the ode was usually performed at a ceremonial occasion, with music. More recently, one of the most glorious symphonic movements of all time, Beethoven’s "Ode to Joy" from the Ninth Symphony, is a musical setting of an ode. It’s a serious and ceremonial form, the kind that might have been sung after a big banquet. Most modern odes still have a very formal, traditional sound. Many of them are poems of praise, or general appreciation.
Nowadays, we often use the phrase "ode to" as an indication of formal praise. As in, "His speech was an ode to his favorite sports team." The Romantics made a big deal out of writing odes to things that most people would never think to praise. like Keats’s "Ode to Melancholy" and Coleridge’s "Ode to Dejection." But the Grecian urn sounds like a pretty cool hunk of marble, and we can see how Keats would want to give it some props.
Some odes follow the formal rules set by the two most famous Greek writers of odes, Horace and Pindar. These poems are called – surprise, surprise – Horatian and Pindaric, respectively. But Keats didn’t follow any set form. The "Ode a Grecian Urn," for example, was borne out of Keats’s tinkering with the sonnet form. In fact, on the page, "Grecian Urn" looks like five short sonnets in a row. But these odes aren’t sonnets, because each stanza only has ten lines, whereas a sonnet has fourteen lines. Nonetheless, Keats’s stanzas have a very tight and regular structure.
The rhyme scheme is fairly quirky, which we might expect from someone who has been fooling around with sonnets. The first four lines of each stanza go ABAB, and except for the last stanza, they are set apart thematically from the rest of the stanza. So, in the first stanza, the first four lines interest the urn and praise its storytelling abilities, and the rest of the stanza begins the process of figuring out what story it is trying to tell. In each stanza, the next six lines go CDE, and then some variation of CDE. For example, in stanza II, it goes: CDE, CED. (If you wanted to write a really interesting and adventurous paper, you might tackle the question of why Keats varies rhyme scheme in the line six lines of each stanza.)
Next, the meter of the poem is in a fairly strict iambic pentameter, where each line has ten syllables, with an unstressed syllable followed by a stressed one. The first line is standard: "Thou STILL un-RAV-ish’d BRIDE of QUI-et-NESS." There are some notable exceptions. In line 13, for example, he begins with a stressed syllable for dramatic effect: "NOT . . ." But, in general, Keats’s meter is so regular that he even instructs us to pronounce the word "unwearied" with four syllables, as un-WEAR-i-ED which sounds old-fashioned.
Although the poem has a deliberately formal sound, it hides its regular structure exceptionally well. You probably wouldn’t even realize how regular the iambic pentameter is unless you were consciously keeping track of the beats. That’s the sign of a great poet at work. Effortless on the surface, extremely complex underneath.