The poem’s first stanza introduces the poem’s main problem and character, builds the setting, and establishes the form.
The poem starts on a tone of lament. In isolation, the opening "O" might just as easily look like the start of a meditation on nature's beauty, or love's glory, but quickly the word "ail" (i.e. "pains" or "afflicts") intervenes and clarifies one of the poem's main themes. This is a poem about pain, and the struggle to understand it. The "O," then, kicks off the inquiry of an empathetic observer. The archaic "thee" gives the reader an early hint about the time period of the poem's setting, one confirmed by the addressee's title in the next clause: "knight-at-arms." Most likely, this poem takes place in the Middle Ages, when one wouldn't necessarily be surprised by the sudden appearance of a knight.
The precise placement of "knight-at-arms" in the line reflects some of the poem's main themes. Knight-at-arms is the knight's formal title. It represents the honorable, chivalrous knight in shining armor. Even the hyphens, which augment the choppy rigidity of the monosyllabic words, contribute to this effect. In the sentence, "knight-at-arms" is isolated by a caesura on one side and an end-stop on the other. Visually and grammatically, the knight is separated from the rest of the action. This has two effects. One, it highlights the tension between the expected composure of an ideal knight and the wallowing of this particular knight, who is a real person with real emotions (the reader will soon learn that the knight is an emotional wreck). Two, it reflects the knight's alienation from his context. As the reader will learn, though the knight loiters by the lakeside, his mind is in a completely different place.
This alienation starts coming through in line 2, in which the knight is described as "Alone and palely loitering." Already the knight is out of context—he's not with other knights, or doing knightly things, but rather alone, seeming to do nothing at all. These words—"alone," "pale," and "loitering"—also hint at some of the poem's main themes. The knight is alone, but was once with someone he loved; he is pale, or reminiscent of death; and he loiters, literally incapable of moving on to the next thing.
The next two lines build on the theme of death, and further establish the setting. The "sedge," or grassy plant life, "has withered from the lake." By using the perfect tense in "has withered," the poem suggests that the withering, though complete, happened recently. The reader, then, might assume that the poem is somewhere between fall and winter. Along with the shriveling plant life, "no birds sing," another sign that winter is setting in. These tokens of death enhance the knight's deathly aspect, and the suggestion of seasonal change signals that the poem will deal with the concept of repeating cycles.
Finally, this jam-packed first stanza establishes the poem's meter. Like other English ballads, Keats works in quatrains, or four-line stanzas, but he adds some of his own touches. The first three lines of every stanza are in iambic tetrameter (each line has eight syllables), and the fourth line of each stanza has four to five syllables.
Stanza 1 follows the meter faithfully, as will the others. It begins,
O what | can ail | thee, knight |-at-arms?
The uniform beats of the unstressed-stressed iambs are appropriate for a ballad, a form that was originally accompanied by music and dance. In this line, the stress falls on the most important words. "What" in some ways prefigures the rest of the poem, as it forms the question the knight will answer; "ail" refers to the knight's pain; and "knight" and "arms" introduce the main character.
The meter in line 4 also serves a special purpose.
And no | birds sing.
This final line kills the rhythm of the three preceding lines. Its first stressed word is "no," giving defeated emphasis to the poem's description of death. As in the poem's other stanza-concluding lines, this one both freezes the preceding imagery with its clipped, terse sound, and, with its monosyllabic speed, propels the poem into the next stanza.