The first four lines of “Mending Wall” establish the poem’s broad concern as well as its form. The poem opens with a mysterious assertion: there is some force that dislikes walls. It spends the next three lines describing the attempts of that force to topple a wall down: it sends frost underneath the wall, causing the ground to swell; when the summer sun later melts that frost, some of the wall's stones become dislodged, opening gaps wide enough for two people to pass through side by side. The force, the thing that “doesn’t love a wall” remains unnamed. The opening lines thus establish a question: the reader is invited to speculate about what the force is, and why it might oppose the very existence of a wall.
This mystery makes the lines feel weighty, as though the deepest mysteries of life are being explored here. But the language of the lines is simple and conversational: except for the compound word “frozen-ground-swell,” the lines contain no words longer than two syllables. These are common, everyday words that anyone might use, perhaps suggesting the universality of the themes that the poem will go on to explore.
There are literary devices at work in the lines, but their presence is pretty subtle. For example, these lines are in blank verse—a literary form with a distinguished pedigree in English poetry. (Both Shakespeare and Milton used it, for example). Although blank verse is an iambic meter, the poem’s first line opens with a trochee:
Something
A reader expects a first line to establish the poem’s meter, and for variations to that meter to come later. But Frost waits to establish his poem’s dominant meter until mid-way through the poem’s first line. As a result, the reader may be slow to identify the meter and to associate it with its prestigious history. Similarly, the first four lines of the poem contain assonance on an /i/ sound ("something," "is," "it," "spills," "in"), but this repeated sound is relatively distributed through the lines: though it binds them together sonically, it does so unobtrusively.
The relaxed, everyday diction gives the poem a casual feel: even as it enters into deep and complex questions, it does so in unpretentious language. It sounds like someone thinking out loud.
This extends to the poem’s use of enjambment and end-stop. “Mending Wall” uses enjambment and end-stop whenever it's convenient, simply following the flow of the speaker’s thought. Despite their use of punctuation, the first three lines of the poem are arguably enjambed and express the continuation of a single idea; only in line 4 does the reader encounter the poem’s first hard end-stop. The opening lines of the poem thus spill down the page, working at the pace of the speaker’s thought.
In this sense, the poem subtly suggests an affiliation with previous poets, like Wordsworth and Milton, who often let long sentences cascade across multiple lines of blank verse. However conversational the language here, the speaker is thus clearly cognizant of deep traditions in the history of poetry and offers subtle signals to readers that the speaker is extending and complicating those traditions.