About a thousand years ago, English began to lose most of its inflections—suffixes that indicated the gender, number, and case of nouns, the tense of verbs, and the person and number of the verbs' subjects. Without the system of inflections for its grammar, English gradually evolved to a fixed word-order to distinguish subjects from objects and the relationships among phrases: Subject-verb-object, S-V-O. Its influence on English speakers can hardly be overstated. S-V-O is our expectation for a sentence every time we hear one or read one, and channels our thinking every time we speak one or write one. Every English sentence occurs against the S-V-O grid: fulfilling it, frustrating it, playing with it or against it. Every English sentence is a mystery novel whose plot we already know. The suspense usually comes not from who did it but what they did to whom, although we certainly want to know who did it right away. If the subject is suspended by introductory clauses, we wait for it, sometimes impatiently, and we want the subject to be the main character, unless we understand from the context why it isn't. We then want the verb to follow quickly after the subject. We want to know what the who did. And we like those actions to be specific, even if they're conceptual or metaphorical actions. If the verb is transitive, we'd like to have the object follow the verb quickly, too. We are eager to understand, to learn, to know. We want our S-V-O fix. That we want it so much allows for all sorts of suspensions and elaborations and fine-tunings of opposite kinds: from James and Faulkner at the one end to Hemingway and Carver at the other.
One could also graph the sentences in poems across the same stylistic range, according to how they fulfill-frustrate-play with-or-against our S-V-O expectation. Poems no less than prose are made of sentences, and expectations of sentences (by the reader), and avoidances of sentences (by the writer). But they are also made of lines that alter our experience of sentences, by foregrounding the sounds of the words, phrases, and pauses which make up sentences but which we don't attend to until these sounds are highly organized and orchestrated. The primary instrument of this orchestration is the lines, and lines can also be arranged in stanzas, which may further foreground the lines by signifying their own organization independent of the sentences. The difference between metered and unmetered lines, in the strictest stanza forms to the free-est verse, is no more than the difference between the degree of foregrounding of the lines against the sentences, and therefore the degree to which our attention to those sentences is complicated.
The sentences are nonetheless always primary. The way to read a poem is to read the sentences (not the lines)—and you parse each sentence as you read it, from phrase to phrase, gambling on the relationships of the phrases, constructing a multi-dimensional Lego, making larger pieces of the smaller pieces as they click into place. We perform this in nanoseconds. We not only understand grammar automatically, we do so at the speed of light, integrating semantics, grammar, and lexicon simultaneously and interactively. When we enter Grammar World we enter inner space at warp speed, brain speed, firing synapses and lighting up neurons still well beyond the understanding of linguistics or neurobiology. We translate sound-bits and nanopauses into words, and group words into phrases. We gamble on the meaning of the phrases, according to not only their grammatical relationships but also the context they create among themselves, and the context of our own individual experience (variable and idiosyncratic as it may be), experience that's being changed even as we read. We interpret, and reinterpret if we have to, and wait to understand. And we provide connections when we have to—among the multiple and variable conceptual, connotative, and metaphorical meanings that words have individually and in conjunction with one another. We provide the mind for the protean organism of language, with its indistinct categories, fuzzy boundaries, inconsistent rules, and bizarre idioms.
Most people learn how to do this before they're two years old. This is why the sentences not the lines of poems are the primary focus of our attention: the function of language is the transmission of meaning, and we've attended to sentences as the life-giving instrument of meaning since infancy, long before we ever started reading poems. But language is also a system in which everything is connected to everything else—tout se tient, in Ferdinand de Saussure's words—which is why the orchestration of sentences through the agency of lines may produce a unique kind of musical meaning that expands the meaning of sentences as they unfold.