As has been said, Duolingo isn't great for language learning. It's okay for vocab/revision, but it doesn't teach you real understanding. I have been learning Dutch through it, but I brought a grammar book alongside, so I could understand what I was doing. Also, I'm an experienced language learner, having spent most of my adult years doing evening classes in one language or another, and I've done bit of teaching Englush as a foreign language.
I'm finding Dutch fairly easy on Duolingo, because I'm a native English speaker and have German to AS-level, and it's somewhere between the two. Without that background, some stuff, like separable verbs, would make no sense at all. But I already know the concept from German, (where I did find them very confusing when I first came across them,) so I just need to learn the Dutch vocab, not the whole grammatical structure.
I think learning any language with no concept of the grammar will be a challenge, because it's the building blocks - learn the pattern and it unlocks a new layer of things you can say. But if you don't understand the patterns, you can't apply them, and Duolingo doesn't really teach that well.