After finishing reading these letters from Nettie—and puzzling over the words in them she does not understand—Celie tells Shug that she again wishes to kill Mr. ____, for hiding the letters away from Celie for so long. Shug tells her not to kill Mr. ____—that would leave Shug only with Grady, and Shug still loves Mr. ____. Celie is excited to have heard from her sister, but so angry with Mr. ____ she can barely think.
Interestingly, Celie becomes aware, through her sister's language, of the vast education she herself has not had the benefit of experiencing. Although it is only referenced subtly in the remainder of the book, it does become clear that Nettie is teaching Celie new words and concepts, however indirectly, through these letters.
Celie says, by way of bargaining, that she won't kill Mr. ____ if Shug can convince Mr. ____ that Shug should sleep in Celie's room from now on. Shug succeeds in doing this, although Celie is not sure how Shug has convinced him.
Again, Shug is capable of getting what she wants from Mr. ____, who has a hard time standing up to her. But it is important to note that Celie is the one behind the idea, and who ensures it will happen with her threat to kill Mr. _____ if it doesn't. Her anger has pushed her out of her habitual passivity. It is not clear whether Mr. ____ suspects, at this point, that Shug and Celie have a romantic relationship.