"To a Snowdrop" begins with the speaker addressing a snowdrop: a small, white flower that blooms towards the end of winter. This flower has apparently blossomed by itself (it's "Lone"), and it's the same color as the drifts of "snows" that "hem[]" it in. But while the snowdrop might look like the snow from which it takes its name, the speaker declares that the flower is "hardier," or stronger. The firm assonance and consonance in the phrase "hardier far" emphasizes the flower's resilience. The snowdrop is a persistent little plant that thrives in these harsh surroundings, year after year.
The speaker addresses the flower directly here and throughout the poem. This apostrophe elevates the humble flower, which will become a symbol of nature's beauty and strength. The speaker also personifies the snowdrop when describing the way its petals droop toward the ground:
[...] once more I see thee bend
Thy forehead, as if fearful to offend,
Describing the flower's petals as a "forehead" brings the snowdrop to life in a new way: it becomes a figure with thoughts and feelings.
The speaker uses a simile to compare the snowdrop's deferential posture, its nodding petals, to "an unbidden guest." In other words, the speaker imagines the flower looks bashful because it's arrived unannounced and is worried it's overstepped a boundary. This suggests the speaker feels a certain affection toward the flower—it may be early, but it isn't unwanted!
The soft /f/ alliteration and consonance in line 3 ("forehead, as if fearful to offend") further suggest the flower's gentility and humility. The snowdrop is a strong, brave flower, but it's not a flashy or rude one. The firm, full-stop caesura in the middle of line 3 (after "guest") evokes the flower's strength as well; the flower's arrival brings the poem to a momentary halt.
"To a Snowdrop" is a Petrarchan sonnet and consists of 14 lines of iambic pentameter. This means that each line contains five iambs: poetic feet containing two syllables arranged in an unstressed-stressed rhythm: da-DUM. As is the case with most metered poems, however, "To a Snowdrop" features some variations on this meter to keep things interesting and to add emphasis to important moments. The first line, for example, actually begins with a spondee (two stressed syllables in a row: DUM-DUM):
Lone Flower, | hemmed in | with snows | and white | as they
But hard- | ier far, | once more | I see | thee bend
(Note, too, that "Flower" should be pronounced with a single syllable, "flow'r," to fit into the meter.) The spondee immediately draws attention to the poem's subject—that solitary snowdrop—while the broader iambic rhythm fills the poem with a steady, familiar march.