A simple solution to Duolingo's problem with teaching grammar.
Duolingo provides little explicit grammar teaching and relies instead on implicit grammar learning.
However, it can be very frustrating to get a translation wrong due to some non-trivial and untaught grammar rule.
So the idea is to make implicit grammar learning less frustrating. (Providing much more explicit grammar teaching would probably cause more users to quit.)
When you are translating a sentence in a lesson, Duolingo could provide feedback in real-time while you type as to how close what you wrote is to the closest correct answer that it has in its database.
You can keep fixing your translation until there are zero errors before you press return.
This feedback would only be provided once Duolingo has determined (in an automated way) that it knows about enough correct translations.