Though wise men at their end know dark is right/Because their words had forked no lightning they/Do not go gentle into that good night.
Wise men, the narrator says, know that death is inevitable and even "right"—it is proper, perhaps, to die and allow the next generation to take its place. The metaphor of the night sky continues, with the speaker referring to death as “dark.” Yet even though death is natural, the speaker asserts that the wise do not readily accept it. Their "words"—their literal speech, but also their wisdom and artistic expression—have not "forked" lightning, meaning that they haven't been able to split a lightning bolt and redirect it, like a lightning rod. Essentially, they haven't left enough of a legacy to last beyond death. The image of the lightning bolt also continues the connection between light and life from the first stanza. Finally, the phrase “good night” reaffirms that death is natural and “good,” even though it is worth resisting, and also echoes the common parting words “good night,” affirming the goodbye death demands.
Good men, the last wave by, crying how bright/Their frail deeds might have danced in a green bay
Thomas adds an additional image here, that of the ocean wave—the wave of the men reaching death—breaking on the shore and dying. Again, he associates light and “bright”[ness] with life. As these men approach death, they realize that their deeds are “frail,” perhaps because they are too weak, due to their age, to leave a stronger mark, and they lament how these deeds could have stood out, like a powerful wave in a calm, green bay.
And you, my father, there on the sad height,/Curse, bless, me now with your fierce tears, I pray.
These lines are crucial and comprise the poem's climax, revealing that the speaker is not theorizing in the abstract, but addressing his father. This disclosure gives the poem personal significance as the speaker implores his father to resist death. The “sad height” on which the father lingers may be a reference to looking down at the Biblical valley of death. The speaker's father's tears would be both a curse—because seeing his father upset would sadden the speaker—and a blessing—because they would show resistance to death. Additionally, the speaker’s plea also suggests that he desires any form of emotion from his father rather than indifference in the face of death, even if that expression of emotion distresses the speaker.
