From my experience, Duolingo teaches you the vocabulary and the set sentences very well. But this is by far not enough. I use regular textbooks that describe the structure of the language, the grammar, the syntax, etc, so as to gain some analytical understanding of it. On top of that, Duolingo helps to get used to recognize these structures and flesh them out with various words. Also, unlike a book, it forces you to listen, and, crucially, to speak. It's a very important step from being able to read written language only to being able to actually talk.