"Who is this person?"
I think it is more sharp. Like, Who is this person?! Rather than, Who is he/she?
Why is this word order allowed, but when I put it the same way for "your father is who?" it was wrong and said the order should be "who is your father".
In Chinese, the position of the question word, like 谁, 什么 is supposed to be replaced in the answer. When used with verb "be", it is generally put after "be".
(Note: This indicates that Chinese can have multiple question words in one sentence. You just need to fill more holes in your answer.)
这个人是谁？(This man is who?) or 谁是这个人？(Who is this man?) are both valid. But the latter is a bit weird.
After replacement, it would be like:
这个人是我。(This man is me.) or 我是这个人。(I am this man.)
If you want to say "Who is he?", it would be 他是谁？(He is who?)
Why is 谁是你的爸爸？ among the tests to be translated to English? That means there are instances where the question word 谁 is placed at the start?
Then why does duo count the latter form wrong some times, and other times act like it's the only way.
Do we really need a measure here? Is Na4 shi4 shei2 not sufficient already?