The first half of the poem is defined by the speaker's use of parallelism, with lines 2, 4, and 6 all beginning the same way:
And make the earth devour her own sweet brood;
[...]
And burn the long-liv'd Phoenix in her blood;
[...]
And do whate'er thou wilt, swift-footed Time
In each of these lines, the speaker addresses time and tells it to do something specific, urging it to force the earth to "devour her own sweet brood," then telling it to burn the "long-liv'd Phoenix," and finally telling it to generally do whatever it wants. This firmly establishes the poem's overall structure, which is based upon the speaker's attempt to talk directly to time.
This parallelism also adds a certain rhythm to the poem, making it feel repetitive and predictable. This, in turn, makes it all the more noticeable when the speaker breaks from this pattern in line 8 by saying, "But I forbid thee one more heinous crime."
Until this point, all of the even-numbered lines begin with the word "And." This one, however, begins with the word "But," signaling a change in the speaker's attitude. Indeed, the speaker suddenly tries to "forbid" time from doing whatever it wants, and this shift is accentuated by the fact that the poem transitions away from the parallelism that defines the feeling, rhythm, and structure of the first six lines.