The author of "Mother Tongue" implies in the excerpt that achievement tests do not adequately measure language ability, as they are too limited in scope and encourage bland, conventional responses over diverse thought.
From the excerpt of "Mother Tongue," we can infer that the author believes that achievement tests give inadequate measurements of language ability. The author criticizes the format of fill-in-the-blank sentence completion, as exemplified by the sentences involving Tom and Mary.
According to the author's mother's perspective, there should be more possibilities for what Tom could be and what Mary might think of him, indicating a belief that such tests are too limited in scope to accurately assess language capabilities and the complexity of thought.
Furthermore, the excerpt implies that the tests are designed to elicit the most bland combinations of thoughts, suggesting that they do not allow for the richness and diversity of actual language use and people's interpretations. This criticism relates to the broader discussion about the effectiveness of standardized testing in gauging a person's intelligence or language proficiency, supporting the view that tests provide an incomplete picture of an individual's abilities.