it would be nice to have mini-lectures available on syntax and grammar. Sometimes you can work for hours experimenting with what the rule at play is until you get it right.
I can't imagine using duolingo without a good grammar textbook to complement. I was lucky to get a great one second hand for really cheap.
Shaum's Outline of French Grammar is cited as the best, and it does seem to live up to the name. You could probably find a pdf somewhere or buy it for cheap.