Okay, Shmoopateers. You're going to have to open the bilingual part of your minds (yes, you have one!) for this discussion of the poem's form and meter. Ready? Off we go…
Here's the deal: the most popular English translation of "A Song of Despair," by accomplished American poet W.S. Merwin, is written in free verse with no real rhyme scheme. It does keep the couplet formation, though, which keeps those lines coming two-by-two until the last couple, which are stand-alone singletons.
The thing is, in the original Spanish version, Neruda had a nice rhythm going that is not conserved in the translation. Every line has fourteen syllables, and every even-numbered line had an A-O (in Spanish, it sounds like "Ah-Oh") assonant rhyme. This means that these lines all end in the same vowel sounds, even if their consonants don't exactly rhyme.
What in the heck are you talking about, you ask? Well, here's an example:
Emerge tu recuerdo de la noche en que estoy.
El rìo anuda al mar su lamento obstinado.
Abandonado como los muelles en el alba.
Es lahora de partir, ¡oh abandonado!
(The memory of you emerges from the night around me.
The river mingles its stubborn lament with the sea.
Deserted like the wharves at dawn.
it is the hour of departure, oh deserted one!) (1-4)
So see how all the lines in the Spanish original have fourteen syllables? Oh, wait. Line 4, which is about being abandoned, only has thirteen? Well, that's likely a little trick to show just how lonely the speaker is feeling—we mean, he doesn't even have all of his syllables!
Also, look at the way obstinado (ob-stee-nah-do) and abandonado (ah-bahn-do-nah-do) end in the same vowel sounds: A-O. This rhyming pattern continues all the way through the poem.
And so, as a result of this choice, we get a sing-song-y approach. While not being too heavily weighted down by rhyme, the song gets a rhythm that does its title proud. After all, we're told from the jump that this would be a song, and the regular meter and rhyme in the Spanish version is certainly in keeping with an expected song structure. Sure, it may not be a song that gets you out of your chair and tapping your feet, but this poem nevertheless seeks to be read like one of those meaningful, sad songs, filled with heartache and abandonment.
What's more, the regularity of the poem's form (in Spanish, that is) really anchors the poem in the natural world that is so important for Neruda. How's that, you ask? Well, the regular rhythm and rhyme pattern can be seen as mimicking the sea and its waves, which is an important part of the natural work in this poem. The addressed ("you") of the poem is associated with the sea throughout this poem, in fact, and it seems that not even the very form of the poem can escape the lost love's influence here. She dominates everything about this poem, even its rhymes and rhythms.