Is it possible to say "un copil" here?
It might be possible, but it does not sound very natural.
The imperfect (citea and era) should be accepted. The perfect sounds more unnatural, in my opinion.
I really hope nobody actually thought that the imperfect would be translated as "he was reading a lot when he was being a child".
The natural construction would be - "El a citit mult in copilarie".
el a citit mult atunci când a fost copil does not work. It was the legit answer in another exercise !!!!
Why "mult" and not "multe"? The previous sentence, I wrote "mult" for "a lot" and it was rejected. And now it's the other way.
Here, "mult" is an adverb, not an adjective. Usually, adverbs take the same form as the masculine form of the adjectives.
And what about "Arta explică multe." ? https://forum.duolingo.com/comment/24854114
It is not an adverb here. You should think this sentence like: Ce explică arta? Multe lucruri. "Mult" doesn't answer to the question "Cât?" anymore, but to "Ce?".
I think the reasoning can be both ways in both sentences.
It does not work. And I am pretty sure other Romanian native speakers will tell you the same thing.
After reading that sentence again, I see what you mean. Thanks for taking your time to explain.
Era copil should also be accepted A fost translates in English into "Had been"