Heartbreak and love
One of the most interesting things about this poem's exploration of heartbreak is the fact that the speaker, while certainly heartbroken, doesn't actually know whether he's still in love. This means that, on the one hand, he might simply be sad because he's missing someone he's in love with—in other words, his heartbreak is a kind of frustrated love with no available recipient. But on the other hand, he may be sad despite the fact that he's no longer in love—in other words, the absence of love is itself heartbreaking for him. To make things even more complex, his feelings of heartbreak seem to make it harder for him to figure out whether he still loves his former lover. His sadness muddles his thinking and exhausts him, making it hard for him to accurately assess his own feelings.
Nature's Indifference
The speaker explains that he is writing his poem on a windswept, starry night identical to one he spent with his lover. He seems to feel perplexed and shocked by how much his romantic circumstances have changed, when nature remains totally unchanging. Because nature remains constant while people change, the speaker is able to make use of the natural world, the wind and the stars helping him assess and measure his changing situation. Yet nature's seeming apathy is also distressing to him. In fact, it appears to make him feel mildly insane or unsure of reality as he attempts to reconcile the peace of nature to the tumult of his own life.
Poetry
The speaker assures readers that he will—or, at least, that he is able to—write the saddest lines of poetry. But, as the poem continues, it begins to seem that the speaker's sadness is not a helpful tool in writing sad poems. Ironically, the speaker's feelings, rather than directly translating to emotive verse, instead impede his expression of emotion. For one thing, he simply seems too sad and tired to express himself with much energy. He's also confused and unsure of his own feelings, perhaps because his loss is still so raw. As a result, he repeats himself and writes in clipped, mild lines and stanzas. Therefore, Neruda suggests, poetry can't be a totally unfiltered record of a writer's moods and feelings, simply because many moods and feelings don't lend themselves to straightforward transcription on the page.
