The first line of “Ode to the West Wind” hints at the poem’s themes and begins to establish its form. The speaker begins his poem by talking directly to the “West Wind.” This is an instance of apostrophe: though the speaker addresses the wind, the wind's not exactly human—and perhaps not even capable of understanding what the speaker's saying, let alone responding to him.
This will be a major problem in the poem. As the poem progresses, the speaker will repeat this gesture, addressing the “West Wind,” calling it “Thou,” over and over (an instance of the poetic device anaphora). As the speaker does so, he begins to question—more and more insistently—whether apostrophe really works, whether it’s actually possible to communicate with the natural world. This is an urgent question for the speaker: he not only admires and celebrates the West Wind, he also wants to share in its power.
In the poem’s first line, the speaker says a couple of interesting things about the West Wind, things that anticipate the poem’s broader themes. First, he calls it “wild.” The West Wind seems out of control—or, at least, out of human control. It is undomesticated, untamed. The alliteration between “wild” and “West Wind” locks in this connection: it makes it seem like wildness is essential to the West Wind’s character.
Then, in the second half of the line (after a caesura), the speaker calls the West Wind “the breath of Autumn’s being.” In other words, the West Wind is the very essence of Autumn, as intimate to it as breath. Autumn is a transitional season, when Summer’s beauty and abundance begin to die out and the bleak Winter approaches. Already in this first line, the speaker suggests that the West Wind is key to that transition—that it is associated with death and decline. (Once again, alliteration—here between “breath” and “being”—locks in the association between the wind and autumn: they seem inseparable).
The first line also introduces the reader to the poem’s form—and its formal irregularities. The poem is written in terza rima. Terza rima uses three-line stanzas, whose rhymes lock together. The first stanza of the poem is rhymed ABA, the next BCB, etc. Terza rima was most famously used by the Italian poet Dante, in his epic poem The Divine Comedy. The form is so closely linked to him that just using it already feels like an allusion to Dante and his epic, which describes the soul's descent into Hell and its subsequent ascent into Heaven. This just suggests that Shelley's poem will also track the soul's journey in some way.
To make matters even more complicated, Shelley arranges each section of the poem into fourteen lines—each section is in terza rima and is a sonnet, albeit an unusual and irregular sonnet. As with most English sonnets, the poem is written in iambic pentameter, but it takes a while to establish its meter: indeed, the first six lines of the poem are all metrically irregular. The first line, for instance, contains a spondee in its second foot, "West Wind"—which gives the line six stresses instead of the usual five. These metrical variations are intentional and important: they make the poem feel as a wild and energetic as the West Wind itself.