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.
[…] when hearing that I have been stilled at last […] (13)
Here's yet another euphemism for death! But instead of the motion implied by "pass" (9), the poet (and his ghostly spirit) has been "stilled."
[…] those who will meet my face no more (15)
This is another way of describing death, but this time, he's doing it through the people who are left behind – the folks who will "meet [his] face no more."
[…] when my bell of quittance is heard in the gloom (17)
The "bell of quittance" is the bell rung in a church tower that marks the death of a person in the neighborhood. Naturally, the bell would be ringing "in the gloom." Again, it's an almost clichéd way of describing death, but like the "dusk" and "nocturnal blackness" above, it's saved from being cliché by the unusual images that come after.