Grammar is not sufficient
I am attending this course but I think it needs a lot of improvement. It needs to introduce grammar but at same time the examples should match with the grammar. I learnt french from Duolingo and this helped me in understanding russian since I managed to obtain a decent grammar book in french about russian language (Grammaire Russe: Les structures de base). I think this book may be helpful in improving the course. Anyway, I think the course is far from optimal (not on par with the french one on Duolingo).