A prose poem is what happens when your normal everyday fiction and your poem love each other very much. They have a baby that looks like prose—it's got sentences, paragraphs, and so on—but reads like a poem, with careful attention to language and rhythm.
"Girl" looks more or less like prose, even though it is the longest run-on sentence ever. But Kincaid's word choice and structure definitely make it a prose poem.
Kincaid didn't make "Girl" a run-on sentence for no reason, you know. Periods are kind of like pause buttons. You get to rest for a bit every time you see one. By taking out the periods, Kincaid makes us keep running along with the text. That's what helps it to sound like Mom is just going on and on and on with her advice.
If you read "Girl" out loud you'll probably notice that the rhythm makes it sound kind of like a song or a chant that kids might say on the playground. Part of that rhythm comes from the constant repetition. How constant? Check it out:
this is how to sew on a button;
this is how to make a button-hole for the button you have just sewed on;
this is how to hem a dress when you see the hem coming down and so to prevent yourself from looking like the slut I know you are so bent on becoming;
this is how you iron your father's khaki shirt so that it doesn't have a crease;
this is how you iron your father's khaki pants so that they don't have a crease;
this is how you grow okra—far from the house, because okra tree harbors red ants (15-20)
See what we mean? Repetitions of “this is how” lulls us into Mom’s world, where all you do all day is cook and clean. It's the rhythm of life, y'all.
On top of that, Kincaid repeats words not only at the beginning but also within and at the end of phrases so that they are almost indistinguishable. For example, lines 15-16 use the word button three times. Normally you’d probably say something like “This is how to sew on a button and how to make a hole for it." And lines 18 and 19 are exactly the same, except "shirt" becomes "pants" (and "it" becomes "they").
You could almost say that, with its rocking rhythm and repetition, "Girl" is an incantation for growing up.
Mom doesn’t only tell Girl how to do things with “this is how” sentences. She also tells her not to do things, saying, “don’t” do something or “you mustn't” do something else; and then she uses either simple imperative verbs or the words “be sure to” or “always."
You might be wondering why, since Kincaid seems to like to repeat things a lot, that she didn’t just begin all of Mom’s sentences the same way. (It's okay if you weren't, because we're going to tell you anyway.) As much as “Girl” is highly repetitive, it also vacillates between nice comfy mom and mean accusing mom.
The sentence structure of “Girl” reflects that. Just like Mom, sometimes the sentences are negative and sometimes they are positive. The repetition and the instructions lull us into feeling that this might be a normal mother-daughter talk with informative content and heart-warming advice, and then, bam! Mom drops the slut-bomb.
This back-and-forth helps us feel Girl's confusion. A list that says a bunch of different things (do this, but don’t do this, do this sometimes, but don’t ever do this, and always do this) is way harder to understand than a list that just says things one way (this is how you do this). So we all get a bit confused by all of Mom’s instructions.
Out of the 53 phrases in "Girl," only three aren't in the imperative mood: “but I don’t sing benna on Sundays at all and never in Sunday school" (14), “but what if the baker won’t let me feel the bread?” (52), and “you mean to say that after all you are really going to be the kind of woman who the baker won’t let near the bread?” (53).
Girl’s words are not imperative because she she's not the one in charge here, Mom is. Notice that both of her lines start with “but." As a conjunction, it connects Girl's interjections to Mom’s words. So while Mom is constantly passing out new instructions, Girl can only reply to what Mom is saying. She doesn't have an independent voice.
Oh, and why is Mom’s last phrase not an imperative? Well, the last two or three lines in “Girl” are a kind of coda that helps bring the text to a close. The non-imperatives break the flow of the text, bringing a list of orders—that seriously seems like it could have gone on forever—to an end. Not only that, but it is the only time that Mom talks to Girl like a person who can think and do things on her own. Instead of barking out orders, now she is the one asking questions.
Although we still get the sense that she doesn’t expect an answer. And unfortunately for us, we never get to hear if there is one.