Do not go gentle into that good night (lines 1, 6, 12, 18)
We know that the first line, the last line, and the title of any work of literature are all extra-important, and anything that gets repeated is probably important, too. So this line, which appears four times in the poem, including first and second-to-last, and functions as its stand-in for a title, must be über-important. Each time it's repeated, it has a slightly different position in one of Thomas's complex sentences, so the meaning of the line changes as the poem progresses. This is one of the amazing things about poetry – the same words mean something different as soon as you repeat them, because repetition and context both change them. In the first stanza, "Do not go gentle into that good night" is an instruction. The verb "Do not go" functions in the imperative mood, also known as the command form. That's fancy grammar jargon for saying that it's a direction: the speaker is addressing a particular person (we'll find out who in the last stanza) and telling that person not to go "into that good night" in a "gentle" way; that is, don't let yourself die easily. Don't go without a fight. In the second stanza, the line appears at the end rather than at the beginning, making it seem more like the final word on the subject than an introductory clause. The verb's mood is also different – here it's in the indicative, which means it's describing something that is happening (most verbs are indicative). Also because it's indicative, it needs a subject – in the first line, the subject was an implied "you." Here the subject becomes "Good men," which suggests that the speaker is contrasting the "you" with the "good men": you shouldn't let yourself die easily, because that's not what good men do. In the fourth stanza, the line has a similar function to its position in the second. It's still imperative, but now the subject is "Wild men" instead of "Good men." We're left wondering: are wild men and good men the same, or are they just overlapping categories? In the last stanza, the verb "Do not go" becomes imperative once again – an instruction, a command, maybe even a plea. You might notice that this is the only time the line appears as a sentence on its own – in every other case, it gets woven into a longer, multi-line sentence. When it stands alone, it gives a sense of closure, of finality: even though the speaker is asking his father to fight death, he seems to know that fight is doomed.
Rage, rage against the dying of the light. (lines 3, 9, 15, 19)
Like line 1, this line is a refrain that gets repeated four times in the poem. Also like line 1, it can either be a command – "Hey, you, yes you over there! I want you to rage!" – or it can be a description – "Good men rage." The "command" happens when it appears as lines 3 and 19, and the description applies to "Good men" in line 9 and "Grave men" in line 15. To be honest, we're a little disturbed by the intensity of this line. The speaker seems to think that the only way to respond to the fact of our own mortality is to get angry, really angry, and stay that way. But that anger doesn't seem to have any real function or use; it's just symbolic of the refusal to accept death. All that simmering rage is frightening – and it's meant to be. After all, the reason that the speaker wants his father to "rage" against death is that he himself feels rage about the fact that he's going to lose his dad. He needs his father to feel the same way about it. Transference, anyone?