In "Do not go gentle into that good night" by Dylan Thomas, what does the villanelle style do for the rhythm of the poem?
The villanelle is a poetic form that consists of nineteen lines: five tercets (or groups of three lines) and one quatrain (or a group of four lines). The first line of the initial tercet is then repeated as the final line of the second and fourth tercets as well as the penultimate line of the quatrain. The third line of the initial tercet is also repeated as the final line of the third and fifth tercets as well as the final line of the quatrain and poem. You'll notice, as well, that these two lines that recur again and again also include an end rhyme: that is, their final words -- "night" and "light" -- rhyme with one another.
In terms of rhythm, the poem is written in iambic pentameter (five feet per line, and each foot contains one unstressed syllable followed by one stressed syllable, for a total of ten syllables per line). Because of the repetition of these lines, as dictated by the villanelle form, as well as the regular meter, the poem maintains a fairly consistent rhythm, lending it a mood of inevitability, as though it is plodding toward an end that it cannot escape. This plodding rhythm and mood is appropriate given the subject matter -- death -- which is certainly inevitable. Although we might be able to hold it off for a while, as the speaker asks his father to do, it comes, as surely as Thomas's next end rhyme, as surely as the repeated lines.