can you explain the difference between the English present subjunctive and the past subjunctive?

-- Midori pg. 232

Imagine watching a bunch of Japanese kids killing themselves, and wondering why they're doing it, so you zoom in on one of them and watch what they're doing. For complicated reasons, they're all walking single-file on a slippery jetty extending out into a vast sea. Waves come along and sweep kids into the water, some swim, some panic, and some sink because they tied weights to their ankles.

Before you know it, you're on the same damn jetty as all the other kids, talking with the guy in front of you. His life isn't bad, it's just that all the kids in front of him had ankleweights on. And he had gotten to know them while they were shuffling along the jetty.

There are lots of ways to look at this: if you know how to swim, it's a nice day at the beach, unfortunately spoiled by some other stupid kids; if you're competitive, it's just a good game of King of the Jetty; but for the dead kids, well, why are we on this fricking jetty, anyway?