Duolinguo has levels dedicated to grammar, and on the web version you also have a grammar explaination for each level.
Memrise is very good if you want to memorize something irregular. For example, in Dutch I couldn’t manage to learn irregular verbs at the imperfect, I found a user created course about them and I have been able to learn them perfectly quickly.
I think I can say that in general the rule of thumb is that Memrise is about memorizing stuff (as the name says) while Duolinguo is more about understand how stuff works (which is why they make nonsense sentences).
But honestly, you’ll never learn grammar well from just apps.
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