Or rather, on just its fourth verse:
(Las tardes a las tardes son iguales.)
and the problem of translating it into English while trying to preserve some of its quality.
Why I find this verse so striking? First, it's an example of a perfect Spanish hendecasyllable, the so-called endecasílabo propio (proper hendecasyllable), with the least number of stressed syllables: the second, the sixth and the tenth, the last two obligatory. Second, the anastrophic antimetabole is masterful. It carries by itself the bleak melancholy of the entire poem, using just two distinct words.
First problem: how to translate "tardes"? In Buenos Aires, tarde is the time of day that stretches from just after noon (1 PM local time) to dusk (5:30 PM to 8:00 PM, local time), with subdivisions like "tardecita" (before 3 PM), "siesta" (3 PM to 5 PM) and "tarde tarde" (6 to 7 PM in summer). Let me try with the generic English "afternoon". Of course, I strive for a iambic pentameter:
The afternoons are equal to themselves.
I dare say it is viable, given the constraints: it is a translation, and it is a iambic pentameter. However, I find it unacceptable: I did preserve none of the qualities of the original. In particular, I miss its sense of "perpetual sameness", the circularity, the eternal return. Second try, a slight variation:
The afternoons are equal one another.
Well, some alliteration here, and to me the nasals are melancholic (No one, Nothing, Never is one of Juan José Saer's novels); however, there is an extra unstressed syllable now that marrs the pentameter. If I drop the requirement of preserving the plural "tardes":
One afternoon is equal to another.
which has a more regular syntax but is definitely a step backwards, and doesn't solve the problem of the extra syllable.
And, besides, the distinct image I have of "tardes" is of dusks, a sun that is forever gone, or rather, the recurrent evidence of the disappearing sun. So how about:
The dusks are equal one another.
Now I'm one syllable short. Again:
The dusks to dusks themselves are equal.
Better, but still short of a syllable. On the other hand, it is not clear if the dusks are equal to themselves (that is, identical as individual dusks), or if they are equal one another, as in the original. Also, I don't like to mix English words of Germanic and Latin origin: I like better using "same" than "equal". So:
The dusks to every other dusk are same.
I badly mangled the syntax (I've found English usage for "is same", mainly journalistic, so I don't think it's unjustifiable, but then); this could be a dead end.
The dusks to dusks are equal and the same.
This attempt leaves me with a sense of accomplishment. Now I understand what Borges meant: it is the days, dusk to dusk, that are equal, not the afternoons themselves. The Jews count days as starting with the first star, that is, at dusk; this sense is nicely conveyed by the English "dusk to dusk". The antimetabole is preserved, and "equal and the same" makes clear that days are not only compared one to another, but confused and confounded into one, unchanging day.