Learn German: Wir wussten nichts über die Hauptstadt. - We knew nothing about the capital.

Wissen is a special irregular verb. In the simple past tense (Präteritum), its stem vowel changes from i to u, and it also adds a -t- before the endings. For wir (we), you add the plural ending -en, giving us wir wussten.

Could I use kannten (from kennen) here instead of wussten?

No, because wissen is used for knowing facts or having information, which fits knowing nothing about the capital. You use kennen when you are personally familiar with a person, place, or thing. If the sentence meant We didn't know the capital (meaning you hadn't been there or experienced it), you would use kennen and say Wir kannten die Hauptstadt nicht.

Doesn't the preposition über normally change between accusative and dative cases? Why is it die Hauptstadt here?

You might remember über as a two-way physical preposition (meaning over or above) that takes the dative for position and accusative for movement. However, when über means about a topic (like talking or knowing about something), it always takes the accusative case. Since Hauptstadt is feminine, the accusative word for the is just die.

How do I know the gender of a compound word like Hauptstadt?