To deconstruct this poem, I'll go stanza by stanza:
What happens to a dream deferred?
Hughes begins his poem with a question, which invites the reader into the conversation. The statement is attention-grabbing not only because of its interrogative syntax but also because of its use of alliteration for the letter 'd': Dream Deferred. In terms of diction, deferred is an interesting choice, meaning "put off to a later time".
Does it dry up / like a raisin in the sun? / Or fester like a sore - / And then run? / Does it stink like rotten meat? / Or crust and sugar over - / like a syrupy sweet?
Hughes begins using simile to compare deferred dreams to other items that could be put off until later or ignored. He compares a dream to a desiccated raisin left too long in the sun, a pus-filled open sore left unattended, ignored meat that begins to smell, and cracked, crusty candy forgotten instead of eaten. Interesting here is Hughes' continued use of the interrogative, which invites the reader to consider each possibility and weigh its legitimacy rather than simply receiving information.
Maybe it just sags / like a heavy load.
This is the first use of a declarative statement, or non-question, in the poem. The tone shifts from questioning to flat, which reinforces the content of the sentence, also discussing something sagging or becoming heavy and unmovable.
Or does it explode?
Switching back to questions, Hughes here uses italics to emphasize the abrupt shift in tone. It passes over the curiosity of the first two stanzas and catapults into hysteria, mirroring the suddenly violent image, totally unlike any other thus far.
An interesting question for the reader is the following: Considering Hughes' personal context, why is the title of the poem "Harlem"?