The opening lines establish the poem's dramatic setting: the speaker is a man addressing his beloved just before they have to leave each other for good. In the poem's first two lines, the speaker is torn between cherishing and extending the blissful present moment as much as possible, and imagining the pain that their inevitable parting will bring.
Lines 1 and 2 share a parallel structure. Both begin with "Ae" (meaning "One"), creating anaphora. More broadly, in the first half of each line the speaker fixes his attention on his interaction with his beloved in the present moment: they share a "fond kiss," they bid each other "farewell." There is a pause (caesura) in the middle of each line, and with the word "then," the speaker looks forward into the future when they must "sever," or separate, "forever."
This parallel structure serves several different purposes. Repeating the same general idea (lovers parting) over two lines emphasizes the importance of this idea, which literally takes up more space on the page. It also allows the speaker to explore and develop different aspects of the idea. The kiss isn't just another kiss, it's a farewell kiss. And this parting isn't just for a week or a month, it's forever. If the reader thought the situation in line 1 was tragic, they realize that it's even more tragic after line 2!
Lines 3 and 4 share a similar parallel structure as well. Both start by describing the bitter passions the speaker will experience without his beloved—"tears," "sighs," "groans"—and both end with "I'll (wage/pledge) thee." By repeating this same phrase at the end of the line (epistrophe), the speaker reinforces the idea that there will still be a connection between himself and his beloved—between "I" and "thee"—even after they have parted.
His anguished tears and groans, his sighs for happier past days, aren't just emotional outbursts he needs to release; he frames them as active ways that he will continue memorializing his beloved. The verb "pledge" often refers to toasting someone and drinking to their health. The speaker transforms his tears into wine in a cup that he will use to toast and honor his beloved. Similarly, the speaker transforms his "sighs and groans" into "wage[s]" he gives to his beloved. He frames his grief not as mere emotional discomfort but as the price he pays for love.
The rhyme scheme reinforces the parallelism between each pair of lines. The poem is written in rhyming couplets, AABBCC, etc. The similarity in sound between the final words of each couplet helps the reader see their similarity in sense. The couple must say goodbye or sever; they must say goodbye forever. The speaker will both pledge and wage his grief to his beloved.
These first four lines also establish the poem's meter. The poem is written in trochaic tetrameter, meaning there are four trochees (DUM-da) per line:
Ae fond kiss, and then we sever;
Ae fareweel, and then forever!
Deep in heart-wrung tears I'll pledge thee,
Warring sighs and groans I'll wage thee.
The meter is extremely regular throughout the poem, which creates a sense of calmness and composure. The speaker is deeply moved, but he not in the grips of uncontrollable emotion that breaks the pattern of the lines. This quiet tone is also enhanced by the line endings. The trochaic meter means that the last syllable of each line is unstressed (a.k.a., a feminine ending), as opposed to the more common iambic meter, which stresses the last syllable. The feminine endings mean that each line sounds as though it's going quiet or dying off as it reaches its end--appropriate for a poem focused on saying goodbye.