"What did the teachers use?"
Translation:선생님들이 무엇을 썼습니까?
사용하다 is strictly "to use" it's also a little more formal and used more in written Korean.
쓰다 is used more in spoken Korean and can mean many different things depending On the context. 편지를 쓰다 (to write a letter) 이 연필 좀 쓰세요 please use this pencil 이 말을 어떻게 쓸 수 있어? How can I use these words?
이 커피가 너무 써요! This coffee is too bitter!
It can also mean to wear something like glasses. 안경을 쓰다 To wear glasses.
Source: Mel (Hinative, Oct 2016)
Why is 선생님들이 무엇을 썼니 incorrect? I was under the impression that it's the past interrogative form of 쓰다 in 해라체? I wouldn't have much gripe about this but both 썼습니까 and 썼니 are in the word bank for this question, if grammatically they're both correct but duolingo only accepts one of them how should I guess which one it is? Just don't put two different forms of the same verb that are both correct in the word bank if you give no indication which one is correct.
They are talking about teachers but not to a teacher so I can't even say I could've guessed by the implied listener. It's fine it just gets frustrating when it happens so often. But let me know if it's marked incorrect for another reason.