Learn French: À cette époque, tu étais studieuse. - At that time, you were studious.

The verb être (to be) is the only verb in French with an irregular stem in the imperfect tense. Instead of using the present tense "nous" form to find the stem, we use the special stem ét-. We then add the regular imperfect ending for tu, which is -ais, to make étais.

How do we know the person being spoken to is female?

We can tell from the adjective studieuse. Adjectives that end in -eux in the masculine form, like studieux, change to -euse in the feminine form. Because the speaker is talking to a female tu, they must use studieuse.

Does À cette époque literally mean "At that epoch"?

Yes, époque is related to the English word "epoch" or "era". In French, the phrase à cette époque is the standard, natural way to say "at that time" or "back then" when setting the scene for a story in the past.

Why use cette instead of ce or ça here?

Cette is a demonstrative adjective that means "this" or "that" for feminine nouns. Because époque is feminine, we use cette époque. You would use ce for masculine nouns, and ça is a pronoun used to replace a noun entirely, not describe one.