This poem is written in terza rima, a form first used by the Italian poet Dante Alighieri in The Divine Comedy. The poem is written in three-line stanzas, which are linked by a rhyme scheme that goes across the stanzas. The first stanza is ABA, the second is BCB, the third is CDC, and the last stanza's two lines are in a DD rhyme scheme. Frost takes out the middle line at the end, ending the rhyme scheme's continuity as he ends the poem.
Frost does not traditionally use terza rima in his poetry, but often experiments with different forms; we think that he has a particular reason to use terza rima for this poem. By moving forward while echoing the past, this rhyme scheme seems to go in circles, just like the moon, an important element in this poem.
Frost also writes this poem in near perfect iambic pentameter. This means that each line has 10 syllables, which are arranged so that one unstressed syllable is followed by a stressed syllable. For example, the stressed syllables in this line are bold and in italic:
I have been one ac-quaint-ed with the night .
This is a very difficult way to write, and it could account for some of the strange sounding ways that Frost phrases his lines. But it's a very important structure for this poem. The steady rhythm of iambic pentameter is like the steady sound of footsteps on the pavement as the speaker of this poem walks around at night.