"Who is your dad?"
I could be wrong in this, but growing up, I perceived a subtle difference (though the translation would ultimately be the same). I usually heard the former in situations where it might be a general inquiry (like, out of all the people in China, which one is your dad?) and the latter in situations where the speaker may have been making some haughty assertion (like, and who, exactly, IS your dad?). Not sure if this is universal though.
This one confused me too, because to me there IS a subtle difference, at lest there is in the languages I know. Usually, whatever comes before the verb is the subject – something we already know and we are talking about – and whatever comes after the verb is further explanation about the subject. So when I see "your father" in the subject position, I assume we already know who he is (as in which one of many other people), and what we're actually asking is who he is, that is what does he do (e.g. whether he's a doctor, or a teacher). But when I see "who?" in the subject position, I perceive it in a different way: that we don't know which person is the speaker's father, and this is the thing we are asking about. We are trying to find a person who matches the description located after the verb. Does this work this way in Chinese too? Or the 是 linking verb is perfectly symmetric and does not distinguish its sides? I suppose it isn't perfectly symmetric, since I've seen sentences like 他是我的爸爸, but not 我的爸爸是他 :q
This exercise accepts both 谁是你的爸爸 and 你的爸爸是谁. The exercise before it (Who is that person-那个人是谁) does not accept the same reversal of sentence structure. Why does it work for one but not the other? Is there some secret rule that duolingo doesn't go into at work here?