but never answers:  "What happens to a dream deferred?"  The images that follow are possible answers to that question and full of sensory details which lead to a feeling of hopelessness. 

Does it dry up
like a raisin in the sun?

Here we have the visual and tactile image of something once full and ripe which is now shriveled and dried up--like a dream deferred.

Or fester like a sore—
And then run?

This image is one of a partially crusted but mostly open sore which has not healed; instead, it oozes its ugly fluids and never really heals--like a dream deferred.

Does it stink like rotten meat?

The image here is one of a life-giving substance which is now inedible and has the look and smell of decay--like a dream deferred.

Or crust and sugar over—
like a syrupy sweet?

Here I think of something like maple syrup left to dry up on a plate--hard and crusty and no longer smooth and usable--like a dream deferred.

Maybe it just sags
like a heavy load.

This image is one of pure weight, a burden which will not be lifted.  Visually, the word "sag" depicts an utter hopelessness--like a dream deferred.

Or does it explode?

Finally, all the senses are invoked in this image--sound and sight and touch and smell and even taste, as the smoke and dust of an explosion fills the mouth.  There is nothing left; all is lost--like a dream deferred.

This structure of rhetorical questions replete with sensory images and details is designed to create a growing sense of hopelessness, which it seems to me is both the tone and the theme of this poem.

"Harlem" is structured as a series of questions about dreams "deferred." These questions are posed in the form of a choice between images. The poem's structure places the emphasis on the images themselves, such as "sore," "rotten meat," and "sugar over," words placed to maximum effect at the ends of lines. Each line ends with punctuation that brings it to a full stop and thus causes the reader to dwell on the final words. The use of alliterative s sounds at the beginning of words also puts emphasis on these particular utterances, such as "sore," "stink," and "syrupy sweet."

The poem's structure emphasizes the unpleasant imagery of aridity, rot, and disease. By doing so, it engages readers in questions about hard shriveled raisins, festering sores, and stinking meat, parts of reality that we usually avoid contemplating. This structure reinforces the bitter and foreboding tone of the poem, as well as the theme that continual hopelessness warps a culture and its people.

The poem is structured, too, so that the last two stanzas are shorter than the long series of images in the second stanza. By setting apart the one-line last stanza, consisting of only four words, the poem's final weight falls on it, heightening its threat: "Does it [a dream deferred] explode?" This leaves the reader with a lingering sense of dread and unease about the conditions in Harlem.