"What did the thief steal?"
Translation:nuq nIH nIHwI'?
You understand it perfectly. This is an instance of Duolingo teaching the wrong thing.
There IS a way to interpret What did the thief steal as non-perfective. It might be asking, what sorts of things did the thief tend to steal? Maybe it was a jewel-thief. That wouldn't be a perfective sentence.
But that doesn't explain why the perfective version, which is the more obvious understanding of the English, isn't accepted.
just to help me understand... this course would have likely translated "nuq nIHpu' nIHwI'" as "what has the thief stolen?". and what you're saying, david, is that either english translation would be fine, because it would only matter that the action has happened in the past and therefore using -pu' or -ta' would be ok?
Right. The translation of -pu' or -ta' doesn't have a one-to-one correspondence to any English grammar. nuq nIHpu' nIHwI' could be translated with any of What did the thief steal? What had the thief stolen? What has the thief stolen? What will the thief have stolen? Context will tell you when the act of stealing took place; the perfective suffix tells you that it's completed from the viewpoint of the sentence. All of these English sentences are perfective, or can be interpreted perfectively: from the temporal stance I am taking in saying this sentence, the act of stealing is viewed as a completed whole.