Una pequeña sugerencia :)
Hi, I'm using DL as a refresher for the Spanish courses I took in school. It's really fun and very helpful in filling in the gaps. One thing that would help a lot would be expanded versions of the underlined explanations for prepositions and other words, like por/para, buen/bueno/bien, a/de/al/del, que etc. This would help explain why one word is used vs. another for a particular sentence. It would also help to know when something is an exception to a rule. I find myself going back to the Internet for explanations. ¡Gracias!
Most folks who spend much time on Duo have figured out which grammar site (I like www.studyspanish.com ) works best for them. Some of the explanations you mention require quite a bit of explaining. ;) I spent two hours in a Spanish class in Mexico just on por/para. Duo is simply not a stand-alone learning platform, and most people supplement the listening, speaking, and grammar with a non-Duo resource.