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Sometimes he is sure she is deficient in understanding, and
at others that her temper only is in fault.

Word:

Deficient.

Question:

Why does he sometimes think she is deficient in understanding and at other times believe her temper is the problem?

Answer (optional):

He sometimes thinks she is deficient in understanding and at other times believes her temper is the problem because Lady Susan has been manipulating his judgment and trying to make him believe that Frederica is to blame for her own actions. Additionally, Miss Summers' testimony contradicts Lady Susan's claims, making it difficult for Reginald to fully trust either side.