"What is behind you?"
Translation:तुम्हारे पीछे क्या है?
The 'kya'/'what' is the subject, not the person spoken to/about. Same as in English.
The verb is always "hai"/"is" no matter who is spoken to/about. "What is behind you?" "What is behind me?" "What is behind them?" If the person spoken to/about was the subject, the sentences would be "What are behind you?", "What am behind me?", ... :-)
Good question, btw. The only reason I happened to see your question is that I brain-cramped, made the same mistake, and opened up the discussion topic before I realized my mistake. :-)
के पीछे is a two-word postposition.
तुम + के == तुम्हारे
तुम + के पीछे == तुम्हारे पीछे
The same thing happens with के पास and के साथ. तुम + के पास == तुम्हारे पास. तुम + के साथ == तुम्हारे साथ.
Not all two-word postpositions start with के. Duolingo doesn't teach it, but "toward" is की तरफ़. And that likewise would combine with तुम to form तुम्हारी तरफ़.
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The above description is practical. Just learn the postpositions that way.
But if you really want to find a ghostposition, पीछा basically means "the area behind someone" (along with other meanings), and the implied ghostposition is "in". तुम्हारे पीछे क्या है == "what is in the area behind you"