This is Francisco de Quevedo's most famous sonnet, and often called the most perfect written in Spanish. It was published in El Parnaso español (1648), three years after its author died; like almost all of his poetry, it never saw print in his lifetime.
The poem argues with a certainty — that death erases everything — and sets an exception to it. The first quatrain concedes the point: the final shadow will come and unbind the soul. The second denies it: that soul will not cross the river of forgetting, because its flame knows how to swim in the cold water. The tercets finish the idea with an anatomy of the lover — soul, veins, marrow — that burns even when reduced to nothing.
It is fourteen hendecasyllables in two quatrains and two tercets, the whole sonnet built to arrive at its last line. Quevedo was a master of conceptismo, the wit that packs a thought into the fewest possible words; here he takes it to the limit. "Polvo serán, mas polvo enamorado" says, in four words, that matter falls apart and love does not — a contradiction that stands on its own.
The verse is kept in Spanish. Translating it in passing would replace the work with a paraphrase of it.