Me too, it's often hard to tell where one word ends and the next word begins.
what would be the difference between "hit the hard wall" and "hit the wall hard"?
"hit the hard wall" - the wall is hard (balla crua)
"hit the wall hard" - the hitting was done hard (bhuail an carr an balla go trom OR bhuail an carr an balla le teannadh)
Remember, Irish doesn't work like English!
Could you say 'bhuail an carr an balla crua go crua'. I know this is not the answer in this lesson and in English it would not be correct, but would the above be ok to say if I wanted to say the car hit the hard wall hard
"Balla crua" crua is an adjective telling what kind of wall. "Go trom" tells how the car hit the wall and is an adverb. FYI, adjectives and adverbs function identically in both English and Irish.
Duolingo needs an entry to allow "struck" to be accepted in place of "hit". The Focloir An Gúm has "hit, struck, beat" as primary meanings and struck is common in topics like this (whereas "beat" is more common for percussion instruments). Unfortunately someone configured the link to not include "My answer should be accepted" (though I have yet to confirm someone actually fixed one of these).
I don't know why you didn't see the option to report a problem, but I can assure you that the course contributors (the folks who create and maintain the course) do review reports about sentences and have added many, many new accepted answers. It really is helpful for future learners to report when an answer you are sure is correct wasn't accepted.