I believe the essay "Mother Tongue" by Amy Tan is an open form prose. It explores the author's incite into the differences in English by native and non-native speakers, and also the perceptions and reactions of others' treatment toward these different speaking types. There is no particular thesis to which Tan is trying to arrive, rather she accounts her journey in understanding that her place in the English speaking realm is culturally different than someone whose family was originating from the United States. She uses a theme-based narrative, and writes in a story-like chronology of her childhood as experiences of growing up with a mother who immigrated from China, speaking English that was considered "limited". She then discussed her experiences as a school girl, and later as a college student who majored in English. Lastly, she tells of her writing fiction as an adult.
Tan writes to a general audience as her tone is informal and she does not assume any particular relationship to her readers. To define the "implied audience", I believe she is writing mainly to native English speakers who have not known the experience of having an immigrant family. In the beginning she says "I cannot give you much more than personal opinions on the English language and its variations in this country or others" (Tan 113). She proceeds to continue in this way to her audience in talking about "all the Englishes I grew up with" (Tan 113).
"Mother Tongue" is a personal essay as Amy Tan delves into her own backround, recounting what she has learned and how she has used this in her present day communication with her readers as well as with her mother. Tan closes with the realization of her own personal success by her mother's verdict of Tan's writing as "So easy to read" (Tan 117).