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.