We just have to keep reporting. "The man who is sleeping." was also marked incorrect.
Duolingo desperately needs a clear KR/EN comparative grammar. At present, student responses are often scored right or wrong at the whim of the composer of the lesson and/or the "teacher" who posts here. If the people of Duolingo cannot state a rule clearly and simply, with examples, then I must recommend against them. I refer not only to this particular exercise but to the coverage of verbal adjectives ("participles," the world calls them) as a whole. Get professional!
How the hell do we know when it's "a" and when "the"??! We're already advanced enough to have a clear understanding. I'm really sick of these ambiguities
You simply don't. If it's given with context, it's easy to figure it out but without any context, both of them are right.
- It doesn't have a verb, yours would require 이다 in some form.
- 자 is not the same as 저