In the book To kill a Mockingbird by Harper Lee, why does Calpurnia enjoy speaking better English than the other church members?
Calpurnia does not "enjoy" speaking better English than her fellow members of her church; she simply knows how. For, in Chapter 12 of To Kill a Mockingbird, Calpurnia reveals to Jem her "modest double life" of having grown up on The Landing, the property of the Finch family. There, Granddaddy Finch given Calpurnia a book to learn from when she was a girl: Blackstone's Commentaries. Since the Bible and this book were the only two that she had, she learned from them. As a consequence, Calpurnia was exposed to Middle English which often had a formal tone and structure.
"That's why you don't talk like the rest of 'em," said Jem...."But you talked like they did in church..."
However, in church Calpurnia chooses to not speak Standard English which she has learned, explaining that it is inappropriate for her to speak "white folks" talk at church, so she adopts the local dialect to be acceptable to the others in the congregation. Continuing, Calpurnia explains that no one would change if she spoke correctly in their presence, so she can either keep her "mouth shut or talk their languages."
This conversation points out just how great the divide is between blacks and whites. And, it also points to the resignation of the black community to be perceived as uneducated and backward. Yet, when Scout is intrigued, Calpurnia agrees to let her visit.