In Shakespeare's Sonnet 19, there is no way to know who is speaking, except that it is someone in love: The speaker could be Shakespeare, but does not need to be. The tone of the speaker deals with the passage of time and all that is lost because of it. It starts out with a sense of defeat and acceptance on the part of the poet, but then shifts later in the sonnet.
The first quatrain personifies time, with an apostrophe, speaking directly to it, and it is capitalized to emphasize its power: "Devouring Time..." The speaker continues by describing things that symbolize power on earth: the lion's paws; teeth from the the tiger's jaws—time can destroy even these. Regarding even the mystical or supernatural, like the mythical phoenix, reborn from the ashes, "said to live about five hundred years," kill it—for "Time" kills all of these powerful things.
In the second quatrain, as "glad and sorry" seasons pass (spring/summer vs fall/winter), do your worst, the poet says; and again, Time, do whatever you want: throughout the world, even with "fading sweets" (flowers); but one thing you cannot touch.
This is the end of the second quatrain, and with the ninth line—the beginning of the third quatrain—the poet's attention to the first two quatrains shifts. He orders, or pleads, that time leave alone the face of the one he cares for; draw no lines of age (with "antique pen"); let this one person's face be spared (remain "untainted") as a pattern of beauty for all that follow ("For beauty's pattern").
The rhyming couplet sums up the poet's main point: Time, go ahead, and do the worst you can because the words written here will allow my love to remain forever ageless. In this way, the writer will be victorious, and not Time.