The passage in question is the first paragraph of chapter 7:
I lived in Master Hugh's family about seven years. During this time, I succeeded in learning to read and write. In accomplishing this, I was compelled to resort to various stratagems. I had no regular teacher. My mistress, who had kindly commenced to instruct me, had, in compliance with the advice and direction of her husband, not only ceased to instruct, but had set her face against my being instructed by any one else. It is due, however, to my mistress to say of her, that she did not adopt this course of treatment immediately. She at first lacked the depravity indispensable to shutting me up in mental darkness. It was at least necessary for her to have some training in the exercise of irresponsible power, to make her equal to the task of treating me as though I were a brute.
The passage is an example of first-person narration, in which Douglass remembers his past. The paragraph is written primarily in the past tense. However, Douglass breaks into present tense when he comments, in the present, about his mistress's past actions, stating "It is due ... to say of her." This indicates that the more rational adult is looking back with mature understanding at a situation that marked his past.
Douglass repeats the word instruct or its variants three times. He uses oppositions when he contrasts how his mistress first acted kindly when she met him to what she became. At first, she "commenced to instruct" him in reading, then she "ceased to instruct" him. Her initial lack of "depravity" in her treatment of slaves is opposed by her later "irresponsible power."
Douglass uses a figure of speech when he says his mistress "set her face against" his being taught by anyone else. "Set her face against" is a visual image that shows how vehemently the mistress turned against the idea of Douglass learning to read. Douglass also uses a figure of speech, in this case, a simile, when he says his mistress started "treating [him] as though [he] were a brute." In this case, he is saying that his mistress learned to treat him as one might an animal.
Overall, in using language words such as depravity and brute, Douglass is expressing his anger at the way he was treated as a child just because he was Black.