The future of the mating of the swallows is predictable because, the speaker says, "dark swallows will return to hang their nests" and the lady of the balcony will witness it through her windows. These are the swallows that "contemplate her beauty and his happiness. These birds new the lover's name; like if they were tender pets that enjoy the love of the couple.
Becquer uses the future tense to cancel the possibility of the swallows going to visit the balcony. The lady will be witness of the swallows mating any swallow. But those that represent the love that the speaker feels for the lady, " those will not return." This second stanza eliminates that promise of living happily ever after and places instead that fallible love. And by using alliteration the speaker gives the reader an image of flowing of feelings. The sound of /w/ is very calming. The dense honeysuckles act the same as the swallows. The poet that is in love admits that the nature is indifferent to his passion and feelings. The honeysuckles