A couple of tips on using Duolingo
I keep open a tab at Quizlet.com in order to add new vocabulary. Whenever I encounter a new word in German I simply add it to a vocabulary set at Quizlet. I make sure to include the article, and I put other important definitions in parentheses. Each set includes about 50 words. Then I use the Learn Mode at Quizlet to help me learn the vocabulary. For verbs I always add the infinitive, 3rd person present if nonstandard, simple past and present perfect. The act of looking up articles and other definitions, and writing this in Quizlet seems to help stick the information in my head.
When a new screen comes up I only listen. I do not look at the written German. Based on listening only, I select my answer. Then I look at the written German to see if I got it right.
Some screens only want a written answer. I listen one time and write my answer. Then I listen again to see if I got it right. If I am really struggling then this won't work.
Hope this helps.