Leda and the Swan
The Petrarchan sonnet is named after Petrarch, a 14th century Italian poet who made the form popular throughout Europe. Like all sonnets, the Petrarchan sonnet has 14 lines. Unlike all sonnets, it also has a major thematic shift after the eighth line. At this point, the poem introduces a new subject or shifts its perspective in some way.
The first eight lines of "Leda and the Swan" describe the act of rape from Leda's perspective. The ninth line, appropriately enough, ends the description of sex. There is nowhere else for the poem to go at this point, so Yeats has to develop a new theme. The last six lines of the poem, then, narrate the consequences of the act, both for humanity (the Trojan War) and for Leda personally (the possibility of "putting on" Zeus's knowledge).
You might have noticed that "Leda and the Swan" looks a little different than other sonnets. It has three stanzas, and line 11 appears to be broken off into two lines. As for the three stanzas, that's a perfectly common way of illustrating the internal divisions in the poem. And we think Yeats divides line 11 in order to heighten the drama of Agamemnon being dead and to show how the poem shifts back to Leda's perspective after leaping forward into the future.
The poem's meter is a very rough iambic pentameter. In this meter, the most common in English, each line has ten beats, with an unstressed beat followed by an stressed one: "He holds her help-less breast u-pon his breast." But only a few lines in the poem conform to this rhythm. For example, the last line has eleven beats. Yeats didn't want to be boxed in by one particular rhythm if he thought something else sounded better. As an exercise, you might try counting the beats and stresses in the rhythmically tight first stanza.
Yeats also plays somewhat loose with the rhyme scheme. The general pattern is ABAB CDCD EFGEFG. But some of the rhymes are only slant rhymes, like "push" and "rush," or "up" and "drop."