"Afero", is the same concept as in catalan "afer"; it is more like referring to "thing" as in "topic", like when you say "we spoke about another thing", or "I have things to do".. that abstract thing. Aĵo is more like an "object" or physical thing.
I've read that afero is usually abstract, while aĵo indicates a physical, tangible thing.
Again, there's some very nice, memory inducing alliteration going on here.
This won't accept "The thing is awful" as an answer. When I input "The thing is awful" it says it's wrong and that it needs to be "That thing is awful", which is obviously wrong. When I insert what it says is "correct" with "That thing is awful", it marks it wrong (as it should) with the correct answer of "The thing is awful." I can't get this one to accept any answer, even the correct ones. Completely bugged on my end.