The poem begins with the simple image of bent birch trees, which stoop toward the ground and create a stark juxtaposition with the other surrounding trees that stand straight and tall. Right away, this contrast takes on symbolic meaning: those "straighter darker trees" represent normality and the boring nature of everyday life, whereas the bent birch trees represent (at least in the speaker's mind) something more interesting and exciting (an admittedly vague idea that will become clearer as the poem proceeds).
To that end, the speaker goes on to say, "I like to think some boy's been swinging them." By saying this, the speaker reveals a desire to view the surrounding environment with a sense of intrigue. Rather than glancing at the bent trees and quickly moving on, the speaker creates an interesting backstory for why they're stooped like this. This, in turn, suggests that the speaker is an imaginative person who looks for evidence of joy and childish excitement in the world.
However, the speaker also acknowledges that the act of swinging from birch trees wouldn't actually make the trees look bent in the way they do now. Rather, "ice-storms" are what cause the trees to stoop like this. By pointing this out, the speaker reveals an awareness of reality—an awareness that ultimately interferes with the more fun, delightful idea of a young boy swinging through the trees. In this moment, then, a tension arises between the speaker's attempt to view the world with the imaginative, curious eyes of a child and the speaker's inability to ignore the much more boring and mundane details of reality.
These opening lines also establish the speaker's use of iambic pentameter, a meter in which each line contains five iambs (metrical feet consisting of an unstressed syllable followed by a stressed syllable, creating a da-DUM da-DUM rhythm). Consider, for example, the first two lines:
When I | see birch- | es bend | to left | and right,
Across | the lines | of straight- | er dark- | er trees
The iambic rhythm of these lines sounds bouncy and consistent, giving the poem's opening a feeling of predictability that is also musical and somewhat lulling—a dynamic that reflects the speaker's contemplative mood.
The first line also features the alliteration of the /b/ sound, which appears in the words "birches" and "bends." This /b/ sound also returns in lines 3 and 4, when the speaker says:
I like to think some boy's been swinging them.
But swinging doesn't bend them down to stay
This use of alliteration gives the lines a strong, rounded sound that is balanced by the softness of sibilant sounds like the /z/ and /s/ sounds, which appear throughout this section:
When I see birches bend to left and right
Across the lines of straighter darker trees,
I like to think some boy's been swinging them.
But swinging doesn't bend them down to stay
As ice-storms do. [...]
The softness of these sibilant sounds gives the speaker's language a smooth, flowing feel. This sibilance might even subtly evoke the sound of branches swishing in the wind.