"What did we talk about yesterday?"
Translation:¿De qué hablamos ayer?
Why was I marked wrong for: "¿De que hablabamos ayer?" In English, there is some overlap between the meaning of "we did talk" (hablamos) and that of "we were talking" (hablabamos). Shouldn't my answer also be accepted, especially seeing as how a conversation can be a continuous activity stretching over an indeterminate span of time?
When asking a question that starts with a question word (like "qué"), or in this case, a question phrase ("de qué"), you have to invert the subject and verb. (i.e., "¿De qué hablamos nosotros ayer?") But there's no reason to use the subject pronoun ("nosotros") when the verb leaves no doubt who is doing the talking, unless you really want to emphasize that you're asking what WE (and no one else) talked about yesterday. Spanish is already heavy on the syllables, so native speakers only use the subject pronoun for emphasis or when it is not obvious from the verb or from context.
Partly because "acerca" would be doing the same job already done here by "de", so it's redundant. It would be like asking, "Of what did we speak yesterday about?" But mainly because Spanish speakers almost universally use the "De qué" + hablar (+ subject pronoun) construction when asking what someone is talking about. I lived with a Colombian family for over a year and never heard the question asked in a different way.