When "explain this" for less than correct answers refers to why the correct answer applies, I think it would be just as helpful if the program explained why the chosen wrong answer is not correct and under what circumstance it would apply. Too hard maybe for "fill the blank" but possible for multiple choice. I think I saw a table that compares these impersonal pronouns, how can I find it? (Algún, alguno, algunos, algunas)
Also, when I looked up the wrong answer I found what I think is less correct English.
Guillermo didn't know many pubs and he wanted to go to some (a random pub). = "Guillermo no conocía muchos bares y quería ir a algún. ALGUNO."
In this case, SOME, implies more than one.