Final answer:
Mistress Hughes stopped teaching Frederick Douglass because her husband directed that educating a slave was dangerous, as it would make him discontented and lead to a desire for freedom. Her actions reflected the notion that education and slavery were incompatible.
Step-by-step explanation:
Mistress Hughes refused to further instruct Frederick Douglass because her husband, Master Auld, had forbidden it, based on the belief that educating a slave was both unsafe and unlawful. Master Auld claimed that teaching a slave to read would render him unmanageable and of no value as a slave. Therefore, her compliance with her husband's directive led to her ceasing Frederick's instruction.
Initially, Mistress Hughes lacked the depravity to deny education to Frederick. But as she embraced the power dynamics of slavery, she developed an aversion to Frederick's learning, exemplified by her anger when seeing him with a newspaper. The idea that education and slavery were incompatible rang true in her actions, further pushing Frederick into mental darkness but solidifying his desire for freedom.