In Sonnet 18, what word signals a message shift in line 9?

Throughout Shakespeare's Sonnet 18, the speaker focuses on how comparing his beloved to a summer's day would not do his beloved justice. In the fourth line, the speaker says, "summer’s lease hath all too short a date," meaning that summer is finite. Summer's beauty does not last, as the speaker says that "every fair from fair sometime declines." Moreover, what beauty summer has is liable to falter: "often is his gold complexion dimm'd." Beauty is fleeting—it cannot last. That is the way of things.

In the ninth line of the poem, though, the speaker's focus shifts. He says, "But thy eternal summer shall not fade." Rather than focusing on actual summer, which is finite, he concentrates on the beloved's eternal summer, which will not fade. The speaker says the beloved's beauty will be preserved "in eternal lines to time thou grow’st," referring to the poem he is writing. It is worth noting that in the eighth line, the speaker says that beauty fades because of "chance or nature’s changing course untrimm'd." Natural things change because they are subject to the passage of time. A poem, though, is not natural; it is artifice. As in many of Shakespeare's sonnets and plays, the artifice of literature or drama captures something more fundamentally true than anything from the natural world would.

What word signals a shift in Sonnet 18?

In the poem's first eight lines, the speaker compares his lover to a summer day, claiming that she is, in fact, far more lovely and temperate than a summer day. He says that, sometimes, the sun shines too hotly and sometimes the sun is dimmed by clouds, and, sometimes, the winds blow really roughly and shake the new flowers. All the beautiful things in nature necessarily fade with time and the changing of the seasons. In the last six lines, however, the narrator says that his lover's beauty, her "eternal summer" (rather than the short, seasonal summer), will never decline, and she will never lose her loveliness. In fact, death will never be able to "brag [that she] wander'st in his shade" because he, the speaker, has immortalized her life and her beauty with these lines. Therefore, he says, as long as men are living and can read his lines, she will continue to live and be beautiful. The word on which this turn happens is the first word of the sixth line, "But." Even the nature of the word itself indicates that it represents a departure from what came before. This turn in the subject is called the "volta," the Italian word for turn.

In Shakespeare's Sonnet 18, it is the word "But" at the start of line 9 that signals a shift in the poem.

Prior to that word, Shakespeare sets up the idea that beautiful things ultimately lose their beauty, and that a moment of perfection is quickly erased.  On a lovely summer day, for example, sometimes the sun gets too hot or goes behind a cloud.

"But," Shakespeare argues, the subject of his poem will neither lose (her?) beauty nor fade away.  In writing about her in her moment of perfection, Shakespeare will immortalize her for all time.  In the final couplet, Shakespeare concludes that as long as there are people on the earth who are able to read, the subject of his poem will live forever in her present beautiful state.

Thus, the "but" in line 9 serves as the transition from the concept of a beauty that fades to one that, through literature, lives forever.