I recently heard poets discuss line breaks and assume that they are easier in free verse than in formal verse. This seems logical since you can break a line anywhere in a free verse poem but in a formal poem line breaks are fixed. However, I would argue that the freedom to break a line anywhere makes it a more onerous task to find the right one.
What purpose do line breaks serve in poetry? The primary purpose is to control pacing, which in turn contributes to the overall musical effect. Line breaks also help to emphasize or deemphasize the significance of a theme or element of a theme. Line breaks help create tension or drama or, contrariwise, create ease and comedy. This is all true whether in free or formal verse. But nearly everything else is different.
With most formal verse line breaks are fixed. That is, with a standard sonnet, you have five feet to complete your line. After the fifth foot, there will be a line break, whether that line is enjambed or end stopped will be a consequence of syntax and diction. What image will be lingering in the mind of the reader as he makes his way to the next line will be determined, again, primarily by how syntax and diction shape the line toward its fifth and final foot. What will confront the reader at the beginning of the next line is also controlled by these two elements of syntax and diction. Manipulation of these two elements is really the only way a line break in a formal poem can be adjusted.
I have written and published a number of formal poems. One thing I learned is that the meter, or more generally, the rhythm, can guide you to the right syntax and diction, or at least to the right syntax. (Diction is not as intrinsically related to rhythm.) The difficulty of line breaks in a formal poem is when the rhythm clashes with the semantic need for significant terms to fall at the end and beginning of lines. But as often as not — or actually more often than not, the rhythm guides the poet to the best syntax for good line breaks. Rhythm, pacing and line breaks play off each other so intimately that one can lead you to the other if you can't concentrate on all three at once. In fact, when I write formal poems, I am primarily led by the rhythm; all the other elements are conjured from it almost as if by magic. This is where a poet writing free verse is at a disadvantage.
In a free verse poem, the line breaks help control the pacing as in a formal poem, however, the line breaks aren’t fixed. The breaks can be made anywhere, theoretically, but to make the best musical effect, they can’t be made haphazardly.
It’s important to remember that the total musical effect is not just a consequence of the rhythm, but is the relationship of every element working in concert: the grammatical structures, the syllabic structures, the phonetics, the syntax and diction, the images and ideas. It’s how all these elements relate to each other and unfold.
Since there is no fixed point at which a free verse poem ends a line, the poet has to listen to the total effect of all its elements to determine each line ending. It’s like needing to hear all the instruments in an orchestra playing in your head to determine what next note the flute should play in the symphony you’re composing. This very difficulty is what made W. H. Auden remark to Stanley Kunitz that he couldn’t write free verse because his ear just wasn’t good enough. Elsewhere, Auden said to Michael Newman, “I think very few people can manage free verse – you need an infallible ear, like D. H. Lawrence, to determine where the lines should end.”
As a regular practitioner of free verse, I find myself constantly revising lines to find the right line breaks. Sometimes I hear them right off and sometimes I don’t. When I don’t, it can take me weeks of trial and error to find the right ones, especially because the syntax and line breaks are independent of each other, unlike in formal verse. In a free verse poem, I could have the right syntax but the wrong line breaks. In a formal poem, if my syntax doesn't create good line breaks, the pacing will be off and I will need to change the syntax. There is an intimate connection among these three elements that can guide you. There is no such echolocation in free verse; all these elements are independent and must be heard distinctly and simultaneously to get them right.