The poem's first line establishes several thematic ideas, as well as certain stylistic patterns that will feature throughout the poem. The speaker begins by wondering "whose woods" he or she is passing through. This demonstrates a concern with ownership and legal rights to land: the woods are not merely the woods but someone's woods. The fact that this is the first issue expressed in the poem also suggests that ownership holds a special significance in the speaker's world. That is, the speaker's preliminary observation directly connects the natural world to the societal one that governs property; already in this first line are seeds of the tension between nature and civilization.
However, the speaker isn't entirely sure of whose woods these are. "I think I know," says the speaker, which calls attention to a level of uncertainty and doubt in the speaker's understanding of the world. He or she does not definitively "know" whose woods these are, but believes he or she knows, which may suggest the speaker's dubious relation to the conventions of society or even the inability to commit to the economy that dictates landownership. In either case, this opening line also establishes a tone of hesitancy that will echo throughout the rest of the poem.
The first line also exhibits the poem's meter. The entire work is written in iambic tetrameter, meaning four iambs, or unstressed-stressed beats, make up an eight-syllable line:
Whose woods | these are | I think | I know.
This pattern is consistent throughout the entire poem, a formal rigor which seems to suggest that the content within is crystalline, perfectly captured and elegantly articulated; this interestingly contrasts with the hesitancy of the line. The speaker seems to be exhibiting certainty about their uncertainty.
The meter also gives the poem a highly musical quality, which is bolstered by Frost's use of devices such as alliteration, consonance, and assonance. "Whose woods" utilizes alliteration in the repetition of the /w/ sound, as well as assonance in the internal /oo/ sound of each word. Alliteration is again evident in the /th/ of "these" and "think." Consonance is also present in the /s/ in "whose," "woods," and "these." All of these features combine to give the line a melodic sound that flows easily off the tongue and reinforces the imagery. For instance, the /w/ sounds in the first two words seem to evoke the whooshing of tree branches or snow in the wind, helping to locate the reader in the forest of the poem.
Lastly, the line is end-stopped: it stands as its own complete thought and does not continue on to the next line. Often, such lines end with punctuation to signal their completion, as is the case with this line. This adds to the sense that the line is declarative and assured, even as it grapples with uncertainty.