«Бро́сил в меня́» usually implies some ill intent: he threw the ball a me, but not for me to catch. In a game context, you would use «бро́сил мне».
"He threw the ball to me again" was marked incorrect. How would that be written in Russian (i.e., to me vs. at me)?
I think "to me" (with the intention that I catch it) would be "мне", but "at me" (without such intention) would be "в меня", as was mentioned by szeraja_zhaba.
Can "бросил" work with an indirect object, i.e. "он опять бросил мне мяч"? Using "в меня" makes the sentence sound like the ball impaled me.
It's actually intentional. :-P As szeraja_zhaba said, «в меня» implies some ill intent on the part of the ball thrower: he threw the ball at you, not to you. Your sentence would mean the latter.
A very poor sentence didactically. One first learns to throw a ball "to" a person before learning to throw it "at" them. It would make the course easier to have a sentence that is a bit more intuitive.
Why not: he again threw a ball at me? Does that have the wrong emphasis?
That is a very awkward English sentence, and wouldn't be considered grammatically correct. The only way I would think to write that sentence would be "He threw the ball at me again"; any other word order sounds weird.
I agree, it is technically grammatically correct, but it sounds awkward for everyday speech, in my opinion.
One more vicious question with only one answer as usual. Everything is done in this course to discourage students.