The poem's enjambments and end-stops are irregular. They fall where the speaker's thought demands them, rather than following a pre-determined pattern. This irregular pattern contributes to the poem’s conversational tone: for a reader, it feels as though the speaker is thinking out-loud, working things through, wrestling with the implications and questions that arise from the debate with the neighbor.
The enjambment in line 25 is particularly evocative, coming as it does after the word "across": as the thought spills over from one line to the next, the structure of the poem undermines the speaker's assertion that the apple trees won't cross over into the neighbor's "pines."
There are also fewer explicitly enjambed lines than there are end-stopped, perhaps reflecting the subject of the poem: each end-stop is like a little wall, while enjambments are the gaps being chipped out one by one. In a poem obsessed with boundaries, smooth, conversational enjambments are potentially thematically important: they provide the reader the experience at the level of overcoming such boundaries. They model, even if only momentarily, the experience of a world without walls.
Recall, however, that enjambment can be subjective; though we haven't highlighted these instances here for simplicity's sake, the first three lines of the poem are arguably better thought of as enjambed—as Frost draping long sentences across several iambic lines so that the reader experiences a pleasurable tension between the finite, contained unit of the line and the seemingly endless unit of a sentence that spills down and down the page. In this sense, "Mending Wall" might seem like one extended thought: the speaker gradually working through the implications of the opening line: "Something there is that doesn’t love a wall…"
Indeed, there are a number of locations in the poem where it is difficult to tell whether a line is enjambed or not. For instance, line 44 lacks punctuation at its end, though one would normally expect it there based on its syntax. Because the poem has used regular punctuation throughout, however, the lack of punctuation here seems unusually loud, like something the reader has to pay attention to. In this moment the poem closely mimics spoken English, which tends to extensively employ run-on sentences. And it again reflects the poem’s broader pattern of using enjambment, a poetic device, to better mimic ordinary speech.