When the Present has latched its postern behind my tremulous stay, (1)
This has got to be the most roundabout way of saying "When I die" that we've ever heard. "Tremulous stay" (i.e., the poet's "stay" on earth, or his life) suggests that all life is fragile, trembling, or "tremulous." The idea of fragile, delicate life connects with the "delicate-filmed" leaves that are described in line 3.
If it be in the dusk […] (5)
This is a more usual setting for describing someone's death – the first stanza, with all its spring imagery, hasn't really set the funereal mood we were expecting. "Dusk" seems less unexpected…of course, Hardy upsets the apple cart again with the unusual imagery that follows, but let's take what we can get.
If I pass during some nocturnal blackness […] (9)
Again, it's a common, almost clichéd way of describing death – we always hear about people "passing away" or "passing on." But here, there's no preposition following the word "pass." He just "passes" – he doesn't "pass away." It sounds like he has just passed someone in a hallway or something. It's as though he's imagining his own spirit "passing" the living after he has died. Oooh, creepy.