Unlike the majority of Shakespeare's sonnets, Sonnet XVIII is written in the Petrarchan form. That is, it is composed of an octave that presents an argument, while the sestet proposes an answer. The argument presented in the octave of this sonnet is whether or not the speaker should liken the lover to a day in summer.
Lines 7-8 present the volta, or turn in thought. That is, the speaker says that everything is nature changes and declines: "...every fair from fair sometime declines." Thus, any kind of beauty deteriorates; it cannot remain the same. For this reason, then, the speaker turns to verse in which to preserve the beauty of his lover:
But thy eternal summer shall not fade,
Nor lose possession of that fair thou owest;
Verse will preserve the beauty that the speaker sees in his love, and Death will not brag that the lover's beauty is falling to death: "wander'st in his shade." This is the solution of the octave.