What is the purpose of a descriptive essay?

Read the excerpt from "mother tongue." those tests were constructed around items like fill-in-the-blank sentence completion, such as “even though tom was mary thought he was ” and the correct answer always seemed to be the most bland combinations of thoughts, for example, “even though tom was foolish, mary thought he was ridiculous.” well, according to my mother, there were very few limitations as to what tom could have been and what mary might have thought of him. so i never did well on tests like that. how does tan build a central idea of her story in the excerpt? tan discusses the types of questions on achievement tests to support the idea that the tests limit students’ ability to write well. tan explains a question on a language achievement test to support the idea that the tests should include more interesting content. tan gives an example of her experience with achievement tests to support the idea that they are not always accurate measures of language ability. tan considers how her mother might answer a question on a test to support the idea that nonstandard english limits a person’s ability to communicate.