For what it's worth, I've also used an app called Lingvist, which tends to tell you the grammar rules more directly. You might like its approach a little better

For what it's worth, I've also used an app called Lingvist, which tends to tell you the grammar rules more directly. You might like its approach a little better. (But also, you might consider getting an old-fashioned grammar book, with tables of declensions and tenses and such, and keeping it nearby while doing Duolinguo exercises.)