The first quatrain of the poem depicts a heavy snowstorm on a winter night. The speaker describes passing by a "field" in which snow and darkness were rapidly "falling." It's not clear what the speaker was doing out in this snowstorm, or where he was coming from or going. (The speaker was probably near "home," mentioned later in line 15, but it's not clear if they headed toward this home or away from it.) The field itself, meanwhile, appears to be rural farmland, with "a few weeds and stubble" showing through a nearly "smooth" blanket of snow. "Stubble" here likely refers to the cut stalks left in a harvested field of grain.
Notice that this first stanza consists of a single sentence fragment. That is, there's no main verb here; the speaker doesn't say, for example, that the snow and night were "falling" or that the ground was "almost covered smooth in snow." These omissions make the stanza feel a bit hasty and perhaps disorienting, as if the speaker is describing the scene in a breathless rush. The words seem to tumble out almost as "fast" as the falling snow and darkness, and the exclamation "oh" in line 1 ("fast, oh, fast") may further suggest that the speaker feels anxious.
The parallel phrasing (and diacope) of "Snow falling and night falling" suggests certain parallels between snow and night: for example, they're both associated with cold, and both are adverse conditions for travel. Nighttime and winter can both be symbols of death, so perhaps the speaker, wherever they're headed, is uneasily reminded that their life's journey will someday end.
The meter of this stanza is somewhat jarring and uneven. The poem is written in iambic pentameter, meaning that its lines generally contain 10 syllables arranged in a "da-DUM, da-DUM" (unstressed-stressed) rhythm. But it takes a while to actually settle into that rhythm; the pattern of stresses in the first two lines, for example, goes something like this:
Snow fal- | ling and | night fal- | ling fast, | oh, fast
In a | field I | looked in- | to go- | ing past,
This unevenness suits the speaker's uneasy, anxious mood, as well as the image of a wild snowstorm. Alliterative /f/ sounds link many of the stressed syllables as well, adding emphasis and bombarding the reader like a "fast" and furious blizzard.
Finally, this stanza establishes the AABA rhyme scheme that will continue throughout the poem. (AABA-rhymed quatrains are known as rubaiyat stanzas, a form explained in more depth in the Form section of this guide.) The three rhymed lines outnumber the unrhymed line in each quatrain—perhaps even seem to overwhelm it, like snow burying the landscape. On the other hand, the unrhymed line, and especially the unrhymed end-word, stands out as unique. In this case, "snow" (line 3) stands out for good reason: it's clearly a key image in the poem!