In one of the flashbacks, Clark recalls Aunt Georgiana teaching him when he was a boy working on Uncle Howard's farm, What subjects did she teach him?

Aunt Georgiana taught Clark Latin, Music, and English/Literature.

Explanation:

Georgiana and Clark are characters in the short story "A Wagner Matinée " by Willa Sibert Cather. When Clark was a young boy, he lived for some time with his uncle and aunt on a farm. Aunt Georgiana is a kind woman who taught Clark while he lived there. Georgiana herself, before eloping and starting this rough, almost excruciating life in the countryside, was a music teacher with better education than many musicians of her time. She gave up her passion to marry Clark's uncle.

In one of the flashbacks, Clark speaks briefly of the subjects she taught him:

[...] would often stand until midnight at her ironing-board, hearing me at the kitchen table beside her recite Latin declensions and conjugations, and gently shaking me when my drowsy head sank down over a page of irregular verbs. It was to her, at her ironing or mending, that I read my first Shakespeare; and her old text-book of mythology was the first that ever came into my empty hands. She taught me my scales and exercises, too, on the little parlor organ which her husband had bought her after fifteen years, during which she had not so much as seen any instrument except an accordion, that belonged to one of the Norwegian farm-hands. She would sit beside me by the hour, darning and counting, while I struggled with the "Harmonious Blacksmith".

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