Duolingo and similar apps do not teach you theory, only words that you will not need in your daily life... My advice, which has worked a lot for me, after different attempts with apps and others, is to read forums in the language you want to learn, if you do not know a word you translate it, that word will be repeated in various readings, little by little you will realize that there are even words that you will learn only by context, without even looking for meanings. On the other hand, don't learn single words, learn phrases to increase your response speed. I wish you much success in your learning!