How does the Nymph's Reply follow up Marlowe's original proposal? What assumption was assumed in the beginning of the poem?
"The Passionate Shepherd to His Love" by Christopher Marlowe is an example of what's called a pastoral. This was a genre of poetry quite common in Marlowe's day, one that presented an idealized view of rural life, an idyll normally populated by satyrs, dryads, cavorting nymphs, and other lesser deities of Greek and Roman mythology.
In the first line, the speaker invites his lover to
Come live with me and be my love
He goes on to present the object of his affections with a sumptuous visual feast of natural beauty that he hopes will entice her to come and live with him:
And we will all the pleasures prove,That Valleys, groves, hills, and fields,Woods, or steepy mountain yields.
And I will make thee beds of RosesAnd a thousand fragrant posies,A cap of flowers, and a kirtleEmbroidered all with leaves of Myrtle
A gown made of the finest woolWhich from our pretty Lambs we pull;Fair lined slippers for the cold,With buckles of the purest gold;A belt of straw and Ivy buds,With Coral clasps and Amber studs
The Shepherds’ Swains shall dance and singFor thy delight each May-morning:If these delights thy mind may move,Then live with me, and be my love.
Unless we get the lady lover's side of the story, then we will remain uncertain as to what will happen. And it's this uncertainty that makes "The Passionate Shepherd to His Love" much more than just a conventional love poem in the pastoral style. It also provides the ideal set up for Raleigh to provide some resolution to the air of uncertainty created by Marlowe. This he does by cheerfully deflating some of the time-worn conventions of the pastoral.
Perhaps Marlowe's luscious Arcadian idyll is all just too good to be true. Certainly the nymph in Raleigh's "The Nymph's Reply to the Shepherd" seems to think so. To come away and live with the speaker doesn't seem like such a tempting offer all of a sudden. Despite Marlowe's best efforts, we are still far from certain as to whether or not his beloved will accept, and Raleigh's riposte more than confirms our initial suspicions. This ravishing nymph doesn't want to spend time with a mortal swain whose looks will one day fade. The unbridgeable gulf between the mortal and the immortal is what keeps the nymph and the shepherd apart, and this is the main thrust of Raleigh's classic rejoinder.
Both poems are examples of “pastoral” poetry, and understanding the conventions of this genre helps us understand these poems. "Pastoral" (from pastor, Latin for "shepherd") refers to a literary work dealing with shepherds and rustic life. Pastoral poetry is highly conventionalized; it presents an idealized rather than realistic view of rustic life, usually with a shepherd telling a shepherdess about his extraordinary love for her. Neither the "shepherd" who seeks a lady's favor nor the "shepherdess" he loves is truly rustic, however; they are "stand-ins" for the poet and his beloved. Because a shepherdess was conventionally depicted as being more free with her favors than would be proper for a well-bred lady, pastoral poetry allowed the poet to imply or reveal erotic unions in a playful way. Full of sexual longing, these poems show the lover's attempts to seduce his mistress or allude to his joy at a consummated union. We find all of this in Marlowe's "The Passionate Shepherd to his Love." Raleigh’s "The Nymph's Reply to the Shepherd" is then a playful response to Marlowe's poem, offering the perspective of a woman skeptical about the shepherd's sincerity who doubts his love will endure if she allows herself to believe his pretty words. She responds to his entreaties one by one.
The Nymph also mentions that IF love would last forever, and also all the things that the Shepherd offers her--the belts and kirtles and beds of roses, etc.--then she WOULD accept. This is true to the carpe diem attitude of the time period, and also to the category of pastoral poetry, to which these poems belong. We know, however, as the Nymph does, that these things do not last forever, so her very realistic answer is "no, I will not come with you and be your love." Raleigh follows Marlow's poem stanza by stanza and matches it in tone, subject, and form/structure.
In Marlowe's original poem, the shepherd proposes an idealilstic love to the nymph. In Raleigh's response, the nymph responds with a realistic response to the details of the shepherd's proposal. The nymph points out that winter will come, hard times will come, flowers fade, clothes wear out, and jewels are simply materialistic. The nymph says in the first stanza and the last that if time stood still and bad things would never come, then she would accept the shepherd's unrealistic proposal.