Structure and context are two great words in literary criticism. Structure means the build of something, how something appears to us in terms of its makeup: what's at the top, what's in the middle, what's at the base. In literary criticism, as in some social sciences like anthropology, structuralists are interested in identifying the elements of a communication, determining logical relationships between them, and finally commenting on the structure depending upon those relationships. Relationships communicate structure. If you see me with a man on the street, walking and talking together, you won't know how to "place" us. But if you know that the man with me is the president of our college, and I am the VP, you would immediately place him higher than me: he above, me below. Structure! A famous anthropologist once said, "Structure is meaning." By analyzing structures you can tell to some extent what they are trying to communicate.
Context is exactly what the word suggests: "con," which means "with" in some European languages, and "text," something that is written. (Actually, the word comes from the Latin "textere," which means to weave. Isn't that interesting? We "weave" the words into a text so as to create something!) So, context refers to the social surroundings within which something is created.
In literary criticism there have been debates about the importance of context. Critics used to believe that you don't need context to appreciate the meaning and the making of a text. Just understanding the words on your own is enough.
I am not one of them. For me, context goes hand-in-hand with the creation of structure—in your case, the question (context!). The context for Shakespeare's Sonnet no. 20 and Christopher Marlowe's Hero and Leander is Elizabethan London, the poets' immediate surroundings, and England and Europe in terms of the literary milieu (i.e., the people, politics, culture, and so on of the times). We need to remember that after the the rule of King Henry VIII (Queen Elizabeth's father), England, especially London, had become "modernized" in comparison to the medieval era. Henry VIII's followers found Roman Catholicism too oppressive, authoritarian, and narrow in terms of patronizing the fine arts. After Henry died, Queen Elizabeth, herself an admirer of literature and a fan of Shakespeare's play, continued her father's open social policies. It is unthinkable that poems like Sonnet 20 would have been tolerated in the pre-Henry era.
So much for context.
Structurally, nothing could be more different between the two poems. Shakespeare's is a sonnet, a fourteen-line poem with metronomic rhyme scheme (i.e., a predictable rhythmic pattern); Marlowe's is full of burning romance, a sprawling piece of (possibly) unfinished poetry. Shakespeare's is terse yet profound; Marlowe's is effusive, passionate. The sonnet is like a painful blood clot; "Hero," an open wound. In my opinion, both are the result of their respective structures.
The structure of Sonnet 20 creates a binary (two things juxtaposed, or set off, with or against each other). In this poem the elements of the binary are in opposition: it consists of a woman that the speaker's friend, a man, loves, and the man himself, object of the speaker's love. Hence, homosexuality is in binary opposition with heterosexuality. The poet is wistful about his love for his friend and slightly disdainful of his friend's mistress, but he accepts the situation.
The structure of Hero and Leander is embedded in the sequence of the story of forbidden heterosexual love because Hero, a Greek nymph and devotee of Venus, is forbidden to love humans. Leander sees her and falls in love, pursues her, and eventually makes love to her after arriving naked at her door. What happens afterward? Marlowe doesn't say. His poem trails off in uncertainty. There is no conclusion. Structurally, the poem is "nonbinding," meaning that Marlowe leaves the end to be imagined by the reader. I think it is a wonderful structural technique, but we are not sure whether the poet intended it to be so or just did not finish the poem. Because its structure is "open," like Renaissance England's comparatively open society, other poets in later years—such as Thomas Hood and Thomas Nashe—attempted to write their own Hero and Leander.
If I were to write this essay, I would first explain what I understand by structure and context. Then I would analyze Sonnet 20, commenting on the two types of love: the speaker's for his friend, and the friend's for his mistress. I could talk of the binary opposition between homosexual and heterosexual love, or between men and women in terms of sexual love. Then I would discuss the structure of the rather long poem Hero and Leander. Because it is long, I would stick to a summary, tracing the story's sequence and emphasizing its inconclusive end. I would comment on the impact of the inconclusiveness of the poem's structure.
If structure is meaning, what meaning emerge sfrom the two structures of Sonnet 20 and Hero and Leander?
You don't have to give a definitive answer. Keep it inconclusive—like Marlowe!