The curfew tolls the knell of parting day, (1)
The theme of mortality appears right from the first line, with the metaphor of the church bell ringing a "knell" for the end of the day. Since a "knell" is a bell toll for a person who has died, the speaker is personifying the day, and is also making death into a kind of universal—it's not just people who die, but even each day dies at sunset! Fortunately, the sun comes up again in the morning, so maybe there's a hint of hope here?
Each in his narrow cell for ever laid, The rude forefathers of the hamlet sleep. (15-16)
The speaker is using a euphemism to describe death here—he says that the dead villagers are just "sleeping." That also sounds hopeful, since after all, if you fall asleep, you're going to wake up again, eventually. Hey, maybe this death thing isn't so bad after all!
The cock's shrill clarion, or the echoing horn,No more shall rouse them from their lowly bed. (19-20)
Oh no, wait! These guys aren't just sleeping, after all—they're never going to get woken up by the rooster crowing again. So much for that hopeful idea. This poem just took a turn for the depressing.