In the original Spanish the poem has a rhyme scheme, and its meter is pretty regular. Each line except for the third, fourth, and fifth is made up of fourteen syllables.
Let's look at lines 5-9 for an example of syllable division:
Son po cos pe ro son…. A bren zan jas os cu ras
(They are few; but they are…. They open dark trenches)
En el ros tro más fie ro y-en el lo mo más fuer te.
(in the fiercest face and in the strongest back.)
Se rán tal vez los po tros de bár ba ros A ti las;
(Perhaps they are the colts of barbaric Attilas;)
o los he ral dos ne gros que nos man da la Muer te.
(or the black heralds sent to us by Death.)
So line 5 has only thirteen syllables, and the rest have fourteen. But also notice that Line 5 has a pause, marked with the ellipsis. Maybe the pause is filling in there for the fourteenth syllable.
There's also a careful rhyme scheme in the original Spanish. In the first stanza, the first and the fourth lines rhyme (ABCA)—but really only because of repetition—and in the following stanzas (like the one above) the even lines rhyme with each other (ABCB). In lines 6 and 8 the rhyme is with "fuerte" ("strong") and "Muerte" ("Death").
This gives the poem a sense of order with little hints of disorder, like the short third, fourth, and fifth lines, the varied rhyme scheme, and the extra line at the end. These hints mimic the "blows" the poem is talking about—they show up unexpectedly to really mess things up for humans, who are just minding their business and trying to build an ordered life.
In the English translation, this poem doesn't have a specific form or meter, which means that the number of syllables in each line varies and they don't rhyme. This is a very modern form of poetry (known as "free verse"), free from all the traditional, more rigid formats. That little act of translation rebellion might reflect on the anger the speaker seems to feel at God and Destiny for letting (or even making) us suffer. Oooh! Take that!
The poem is made up of four stanzas of four lines, each with one extra line hanging off the end. This makes it kind of look like a sonnet (which has just three four-line stanzas, and only fourteen lines). More important, this shows that the poet was paying attention to form.
The last line ("There are blows in life, so powerful…I don't know!") is separated out, as though it were its own little stanza, which really makes us focus our attention on it. The fact that it's off by itself, with no other lines to protect it from the big, bad world, kind of reminds us of the speaker. The speaker feels like the world and its problems are pretty chaotic, and don't follow the nice order of a sonnet—or a justice system, say.