How do we know that the subject is " you " ? Why can't it be " what do we do now ? " or " what to do now ? " for example ?
Word endings are important. We have a variety of verbal endings which can be helpful to know who does what. They are NOT fixed as in Spanish but work great with conversational contexts.
"지금 뭐 할까?" = What shall we do? or What do you want me/us to do?
"지금 뭐 하나?" = What shall I do? (talking to oneself)
"지금 뭐 하니?" = never used for 1st person.
"지금 뭐 해?" can be used for all grammatical person, but without a certain context it is used for 2nd person.
How do you interpret "What to do now?" in contexts? You can get some hints.
"Now what are you doing." has a different nuance. Rather than pure curiosity it implies concern, skepticism, disapproval, puzzlement, etc.
We don't know for sure what /who the subject is without more context. It could be "I" or "we" in a moment of frustration. "What (the heck) are we doing now?"
Or it could be someone we just referenced. (he, she, they)
"I ran into 절수 on the way.
그래? 지금 뭐 해? Oh yeah? What's he doing?
It's interesting that using the progressive form was considered wrong for this grammar structure earlier in the course, but at the moment "What do you do now?" is considered wrong.
It seems like they aren't making corrections at the same pace they were at the start.