What poetic devices are used to support and enhance the theme of loneliness?

“Acquainted with the Night” uses the terza rima form and repetition to formally produce a sense of isolation. This isolation is haunting, and it is experienced by the reader as an endless cycle, one in which there is little more than the “I” of the speaker.
“Acquainted with the Night” consists of four stanzas of three lines and a final couplet. These three-line stanzas feature an interlocking rhyme scheme that is known as terza rima. (This form is often attributed to Dante Alighieri, who used it in The Divine Comedy; it is fitting for a poem about isolation.) In the terza rima, the second line of each stanza is isolated with its rhyme withheld until the following stanza. While this produces a sense of progress—and as we read we might imagine this as a progress away from isolation and toward another person—the final couplet frustrates this trend. Instead, we are brought full circle, returning to the original rhyme from the first stanza (“night” and “light”). What makes this return particularly devastating—and further emphasizes the sense of isolation—is that the poem concludes with the same word from the end of the first line: “night.” The speaker is still in the darkness; or as it is put in the second line of the poem: “I have walked out in rain—and back in rain.”
Whereas the terza rima form suggests progress and movement, we discover that this is simply another walk there and back again. The abundance of “I” is evident in the first half of the poem, starting as it does six of the first seven lines. While “I” does vanish almost entirely after line seven, the return of the rhyme scheme produces a circular motion, reminding us that “I” is still all that there is. It is fitting then, that the return to “night” in the final line of the poem is also a return of the “I”—the speaker’s utterance of isolation.