The poem begins in a landscape without human beings. The first line tells readers that "men were all asleep," a fairy-tale-like image in a big city (where, in reality, you could count on at least someone being awake all night!). This line sets the stage with a sense of mystery and enchantment, later emphasized by the speaker's description of the snow as "stealthy": this snow falls unseen by humans (and perhaps could only fall unseen by humans).
Much of this stanza focuses on the snow's action, describing the way the snow moves and gathers. The speaker evokes the snow's accumulation through the use of the past progressive tense (which is the technical way to describe the use of verbs ending in "-ing" in the past tense). Rather than saying that the snow "flew" or "fell," the speaker says that it came "flying," "falling," etc. The feeling of ongoing action these "-ing" verbs create suggests a steady, accumulative transformation, not an instantaneous one. The reader thus gets a private window onto a slow process that the sleeping Londoners only see all at once after the city's transformation is complete.
The snow performs three major transformations in the poem: it makes the city quiet where it was noisy, white where it was brown, and even where it was uneven. The speaker's use of alliteration and sibilance in these opening lines helps to build a picture of the smooth silent landscape the snow is creating: the softness of /s/ sounds and the regularity of repeated initial letters mimics the snow's gentle, inescapable unification of the city. In addition to sibilance, note how the frequent consonance of /w/, /l/, and /f/ sounds throughout these lines adds to their sleepy, gentle, muffled quality:
When men were all asleep the snow came flying,
In large white flakes falling on the city brown,
Stealthily and perpetually settling and loosely lying,
Hushing the latest traffic of the drowsy town;
The speaker will repeat these sounds throughout the first nine lines of the stanza. There is a great deal of assonance throughout these lines as well. Note the short /eh/, long /ee/, /ow/, /ay/, and long and short /i/ sounds:
When men were all asleep the snow came flying,
In large white flakes falling on the city brown,
Stealthily and perpetually settling and loosely lying,
Hushing the latest traffic of the drowsy town;
All together, these lines are intensely musical—filled with repetitive, incessant sounds that reflect the hypnotic nature of the falling snow.
Finally, both the snow and London are personified here and throughout the poem. The snow has agency—the ability to be "stealthy," for instance—and London is described as being "drowsy" because it is nighttime. This personification reflects the poem's thematic idea of unity, presenting both the snow and the city itself in this moment as cohesive, unified entities.