Questions & Answers about Моя тётя была бухгалтером в банке.
Why does the word for accountant end in -ом here?
It is in the Instrumental case (бухгалтером). In Russian, when you state someone's profession using the past or future tense of "to be" (like была), the profession noun must take the Instrumental case.
Since тётя (aunt) is a woman, shouldn't the word for accountant have a feminine ending?
Many profession words in Russian, including бухгалтер, врач (doctor), and инженер (engineer), are masculine nouns that do not have a widely used feminine form. You use the masculine form even when referring to a woman.
If the profession word is masculine, why is the verb была feminine?
The verb always agrees with the subject of the sentence. The subject here is тётя (aunt), which is a feminine noun, so the past tense verb must be была. The gender of the profession word does not affect the main verb.
Why do we say в банке instead of в банк?
Because the aunt was located inside the bank. В банке is the Prepositional case, which is used for location (answering "where?"). If she were traveling to the bank (motion), you would use the Accusative case (в банк).