Introduction:
Zitkala-Ša, also known as Gertrude Simmons Bonnin, was a Native American writer and activist. In her autobiographical work, "The School Days of an Indian Girl," she highlights the difficulties she faced while attending a boarding school for Native American children. One of the challenges she encountered was the strict rules and regulations surrounding mealtime, which she found particularly challenging.

Explanation:
1. Inability to follow the bell taps:
Zitkala-Ša found eating by a formula a hard trial primarily because of her inability to follow the bell taps. At the boarding school, meals were regulated by a strict schedule. The ringing of a bell signaled the beginning and end of each mealtime. However, Zitkala-Ša, being accustomed to a different way of life and cultural practices, struggled to adapt to this new system. She was not familiar with the concept of bell taps and found it difficult to understand and follow the instructions given through the ringing of the bell.

2. Confusion and lack of understanding:
The use of bell taps to regulate mealtime was a foreign concept to Zitkala-Ša, and she found it confusing and challenging to comprehend. The lack of clear understanding and familiarity with this system made it difficult for her to know when to go to the dining hall and when to leave. This confusion added to her overall struggle with eating by formula.

3. Cultural differences and loss of freedom:
The strict rules and regulations surrounding mealtime at the boarding school also represented a loss of freedom for Zitkala-Ša. In her Native American culture, mealtime was a communal and flexible activity, where individuals ate when they were hungry and stopped when they were full. However, the boarding school's formulaic approach to eating imposed rigid rules and restrictions on her, taking away her autonomy and cultural practices.

Conclusion:
In conclusion, Zitkala-Ša found eating by formula a hard trial primarily because of her inability to follow the bell taps. The unfamiliarity with this system, confusion, and lack of understanding made it challenging for her to adapt to the strict regulations surrounding mealtime at the boarding school. Additionally, the loss of freedom and cultural differences further contributed to her difficulties in accepting and embracing this new way of eating.