Still not accepting "when" for "agus" as of July 20, 2016. Quite annoying seeing the course has sentences where you are expected to use "agus" as "when".
It doesn't accept "when" because the sentence doesn't mean "I fell when the car hit me".
"I fell when the car hit me" means that the car caused me to fall - thit mé nuair a bhuail an carr mé.
"I fell and the car hit me" means that I fell first, and then the car hit me. Thit mé agus bhuail an carr mé
agus could be translated as "when" if it is clear that the fall occurred first - "I had fallen when the car hit me" (you can replace "when" with "and then" in this sentence, as scilling suggests above). Bhí mé tar éis titim agus bhuail an carr mé