"She was a bad teacher."
Translation:Ona byla špatnou učitelkou.
My answer of "Ona byla špatná učitelka" was accepted, but I am wondering why it is seemingly okay to use either the Nominative or the Instrumental case here? Is one more correct than the other? This seemed like a very straightforward Nominative sentence for me, so I am afraid I don't entirely understand when the Instrumental should be used (aside from prepositions that require it, and when there is an instrument with which something is done).
These cases coexist for this type of phrase and you can generally use both. There is a small distinction, but it is such a fine point that even the native speakers often do not know the details. The instrumental (for a noun) is better for something temporary and nominative for permanent or inherent facts.
Actually, although it may seem like a straightforward nominative sentence for a speaker of English, speakers of Polish will say the exact opposite.
Thank you, this explanation was helpful! I've been informed that https://cs.wikipedia.org/wiki/Nominativ#Konkurence_nominativu_a_instrument%C3%A1lu also explains it quite well, but is only helpful if you can already read Czech...
It also makes much more sense now why this skill is stressing the use of the instrumental so much (in more ways than just occupations)!