Here's why it's open to interpretation:
* The speaker is alone: The speaker is clearly by himself, as the poem is in the first person. There's no mention of anyone else being present.
* The line's context: The line "Whose woods these are I think I know. His house is in the village though; He will not see me stopping here To watch his woods fill up with snow." suggests the "they" refers to the owner of the woods, a "he."
* Ambiguity is intentional: Frost often uses ambiguity in his poems. The mystery of "they" adds to the poem's sense of isolation and the speaker's internal struggle.
Possible Interpretations:
* The owner of the woods: As mentioned above, this is the most common interpretation.
* A personified entity: The "they" could represent a larger force, like nature itself.
* The speaker's own thoughts: Some readers interpret "they" as the speaker's own thoughts or desires, pulling him away from the solitude of the woods.
Ultimately, the beauty of the poem lies in the fact that there's no definitive answer. The reader is left to ponder the mystery and draw their own conclusions.