The text does not directly reveal the reasons for Ha's regret once she begins to learn English. However, a close reading of the text will provide a clearer picture.
Ha's interesting story is told in a succession of poems. She is a 10-year-old girl who fled her home country with her mother and brothers during the Vietnam War. In the poem "Missing in Action," we learn that Ha's father was captured by the North Vietnamese while on a navy mission. He is never heard from again.
The poems also discuss how Ha comes to learn English. In "English Above All," Ha's regret in learning English is implied.
Until you children master English, you must think, do, wish for nothing else. Not your father, not our old home, not your old friends, not our future.
The words above are supposedly spoken by Ha's mother, who is emphasizing the importance of learning English. The entire family will need to speak English to function well in American society. However, Ha regrets the new lessons because her mother is behaving as if it is the most important thing in the world. It is difficult for a 10-year-old girl to forget her missing father, her old home, and her old friends. Essentially, the English lessons threaten Ha's sense of self; the familiarity of the past is being replaced by the ambiguity of the present. Ha's mother implies that even thoughts about the future are out of reach without fluency in English, the language of Ha's new country.
Additionally, Ha feels regret when she starts to learn English because the language is difficult to master. She becomes frustrated by strange grammar rules; this is made clear in the poem "Third Rule." Another reason Ha feels regret is that other students assume she knows nothing because of her poor grasp of the English language. This is made clear in the poem "Feel Dumb."