Ralegh's poem continues a discussion between a shepherd and his lover begun in Christopher Marlowe's "The Passionate Shepherd to his Love," using a similar metrical scheme and imagery, but arriving at a vastly different conclusion.
One of Ralegh's techniques is to juxtapose the ideal--the shepherd's desires--with the nymph's relatively somber view of reality, creating a constant tension between the ideal and the real. The first stanza, for example, begins with an ideal, but conditional, scenario:
If all the world and lover were young,/And truth in every shepherd's tongue,/These pretty pleasures might me move/To live with thee and be thy love.
The Nymph's message, unfortunately, reflects her conviction that 1) the world and love are no longer young and 2) truthfulness, especially when trying to convince his lover to be with him, is not the shepherd's greatest attribute. The stanza also employs an important poetic technique--alliteration, the repetition usually of initial consonants--to move a line smoothly along: "pretty pleasures might me move."
Unlike the shepherd in Marlowe's poem, who sees life and love in the ideal time of spring and summer, the nymph expresses a much somber and realistic view:
Time drives the flocks from field to fold,/When rivers rage and rocks grow cold;/and Philomel becometh dumb;/The rest complains of cares to come.
She refers, of course, to the world in winter when flocks must be protected, rivers become dangerous, rocks are no longer warmed by the summer sun, and all the world prepares for the harsh reality of winter. Ralegh's allusion to "Philomel becometh dumb" reminds his readers of the Greek myth in which a beautiful girl named Philomela is kidnapped by her sister's husband, who falls in lover with and then cuts out her tongue to insure that she doesn't tell anyone of his abuse of her, and as he was about to kill Philomela and his wife, Procne, all three were changed into birds--Philomela into a nightingale. The nymph's purpose here is to reiterate her belief that "truth in every shepherd's tongue" cannot necessarily be relied on.
As we move through the third and fourth stanzas, the nymph provides a catalogue of the destructive effects of winter and time, using balanced diction and images to carry her point:
A honey tongue, a heart of gall,/Is fancy's spring, but sorrow's fall.
We have in these lines very clever juxtaposition of opposites--honey/gall and spring/fall--as well as a wonderful play on words. In "is fancy's spring," Ralegh uses spring to indicate both the season and the physical image of imagination springing forward. "Sorrow's fall" describes both the abstract concept of sorrow governing one's mind and being associated with the season of fall, a continuation of the nymph's belief that life and love do not exist in a perpetual spring and summer but are also affected by the harshness of fall and winter.
The last stanza answers the first--if youth and love were permanent, and age and joys had no expiration date, then the nymph would consider ("might move") living with the shepherd and sharing his love.
In essence, we have the juxtaposition of an innocent or naieve view of life and lover, represented by the shepherd, and the nymph's serious, sober view of the realities and harshness of life. At the end, however, she at least concedes that if conditions were different, she would be willing to share her life and love.