Well that was an astute review of the piece below: Wow. Let's see if I can be a bit more specific.
The language (at least in translation) is simple and direct. Neruda uses a metaphor for time as two rivers, the past flowing backward, the other flowing forward, though moving with you. So this is not the future precisely, but the possibilities ahead. The present is that "single second they may be joined." The imagery of the river is a bit more powerful than I picture in reading this passage. I imagined a quick-running stream, where I could visualize my bare feet on the rockbed, the water towards me in one direction only, yet where it splashes against my legs, it springs forward before it falls back into the water. That, I see, as that "drop of an instant."
While my interpretation doesn't follow Neruda's two rivers, both in size and direction of flow, I would likely note attitude and experience influence the image. In this case, I see myself walking upstream, not necessarily fighting the flow, but rather seeking its source. Personally, I'm not a strong swimmer and that's probably why I made the rivers into streams, feeling more secure and able to navigate my way.
Neruda makes a point of the instant the present becomes the past: "Now. This is that moment, the drop of an instant that washes away the past. It is the present." What a wonderful image. A "drop" of a riverful of water; as I type this it just became something I've typed. "Racing, slipping."
There is no regret in Neruda's poetic assessment; there is hope and instruction to use time with love and care. "Time that flows will have the shape and sound of a guitar," he says. Here I listen for the hum of life, the natural sounds that include silence as a song.
Neruda leaves us with this: "Time is joy.
The Lost Children: A Charity Anthology