The speaker uses anaphora in the sonnet's opening octave to emphasize the point that Black people will not have to endure racist oppression forever. Starting in the first line, the speaker says, "We shall not always plant while others reap." Then, two lines later, the speaker uses a pared down version of this sentence construction by saying, "Not always countenance[...]." This time, the speaker leaves off the words "we shall," which are implied because they appear at the beginning of the poem. In turn, the speaker is able to place extra emphasis on the words "not always," thereby underscoring just how hopeful the speaker is that oppression will not last forever.
The speaker continues in line 5 by using anaphora to repeat another variation on the phrase, this time saying, "Not everlastingly[...]." For something to be "everlasting" means that it lasts forever. As such, the use of the word "everlastingly" is little more than a replacement of the word "always," ultimately enabling the speaker to build upon the stanza's central idea that the misery that comes along with racism and oppression will someday end. Then, in line 7, the speaker returns to the words "not always," saying, "Not always bend to some more subtle brute."
All in all, the speaker's use of anaphora stresses the idea that the oppressive conditions in which Black people live are temporary. This is an especially important idea when considering how long racist oppression has been present in the U.S. As a result of this terrible history, it might seem like Black Americans will never experience what it's like to live freely. The speaker, however, is confident that the day will come when Black people are able to triumph, and this is why the speaker repeats the words "not always" so many times, putting a sense of hopefulness on display in a way that is uplifting and encouraging.