This poem by Robert Frost is a sonnet: 14 lines of iambic pentameter. It uses rhyme and rhythm, sound devices, figurative language, and ambiguity in intriguing ways.
First, although it is a sonnet, it uses a rhyme scheme that is not typical of the form. It consists of four tercets (three-line stanzas) followed by a rhyming couplet. The rhymes are interlocking: The end rhyme of the second line of each stanza becomes the rhyme of the first line of the following stanza. In this way, each successive stanza turns the poem back on itself in a revolving pattern, matching the way the poem's speaker "walked out in rain -- and back in rain." Ending the poem with the same line as it starts with uses repetition to bring the poem full circle, just as the poem's speaker has made a round-trip journey in the rain.
Although the rhythm is theoretically iambic (an unstressed syllable followed by a stressed syllable), in actuality one reads the lines that begin with "I have" with a stress on the "I." This creates a faltering rhythm consistent with the tentative spirit of the poem where things seem "neither wrong nor right."
The sound devices used include alliteration and consonance. The most prominent examples of alliteration (repetition of initial consonant sounds) are: "stood still and stopped the sound of feet," "saddest city," and "neither wrong nor right." The most obvious example of consonance (repeated internal or end consonant sounds) is the liberal use of the "t" sounds throughout the poem, including: acquainted, night, walked, outwalked, city, light, looked, saddest, passed, beat, dropped, still, stopped, feet, interrupted, street, still, height, against, time, right. Other oft-repeated soft consonants like the "s," "l," and "w" sounds give the poem a hushed tone consistent with the quiet, rainy evening the poet describes.
Line 12 contains a metaphor: "One luminary clock against the sky" compares the moon to a clock.
Ambiguity permeates the poem, creating an air of mystery and uncertainty. The cry the speaker hears is "not to call me back or say good-bye," but we are not told what it is. We know the time is "neither wrong nor right," but for what, we wonder. We do not know why the speaker has walked out at this time of night, nor do we know why he would drop his eyes and not want to explain his trek to the night watchman.
The rhyme and rhythm, sound devices, figurative language, and ambiguity of this poem make it both soothing and haunting.