That's the topic "cases", and I don't think you can blame the book for not explaining Duolingo's shortcomings. In the book cases logically have to be a little ways after the introduction to articles, don't you think? ...Maybe not a little ways. That's actually a pretty complicated topic, especially the "einen" - "einem" part... I mean, I don't know, how does it sound to you to put Duolingo aside for a couple weeks until you catch up in the book? Or do you think you're going to lose motivation? That really seems to be turning into an excessively convoluted process.

On the other hand, I'm personally am not particularly bothered by grammar in Welsh for now, because I'm trying to first get a good base of root words, so for one exercises I come across won't be too frustrating (because of not knowing the words and not understanding the grammar at the same time). But in Duolingo you don't really get the option to ignore grammar, because it generally doesn't accept grammatically incorrect sentences...