Learn Polish: Wówczas owa anegdota była bardzo znana. - Back then, that aforementioned anecdote was very well-known.

owa is the feminine nominative form of the formal demonstrative pronoun ów. While ta just means "this" or "that", owa specifically means "that aforementioned". It is highly formal and literary, used to refer back to a noun that was already established in a previous sentence.

Does the ó in ów change when forming the feminine owa?

Yes! The base masculine nominative form is ów (pronounced with an "oo" sound). When we add endings for other genders and cases, the ó shifts to a regular o. This is a common vowel shift in Polish. That is why we have owa (feminine), owe (neuter), and owego (genitive), all pronounced with a standard short "o".

Is there a difference between wówczas and wtedy?

Both mean "then" or "at that time" and are interchangeable in meaning here. However, wówczas is slightly more formal and literary. It pairs perfectly with the elevated, written tone set by the demonstrative pronoun owa.

Why is znana in the nominative case here rather than the instrumental?