So, it has been a while since I used Duolingo, and I have been trying to get back into it, just for personal interests.
What happened to the flash card thing that used to be here? That was nice to have, since remembering specific words are where I do the worst.
Look here: https://tinycards.duolingo.com/
Even with its flaws it's much better than the old system here on the site.
the flashcard feature has been replaced by "tiny cards". i was not on duolingo when it had the flashcard feature so i cannot tell you the differences. i can tell you however that "tiny cards" is set up well and that i love it!
Over the past year I've found that Duolingo doesn't do a very good job of teaching the words. For German nouns it is especially critical to learn the gender in order to build the rest of a sentence correctly. I keep my tree golden but about 20% of my Words are stuck at 'Overdue' no matter how much time I spend on Duo. I can see the words and their strengths at https://www.duolingo.com/vocabulary/overview. Even though Duo claims to make you practice words to get them into long term memory, there are many words that it never practices. I don't have any confidence that Tinycards teaches the words any better.
Instead, I started making real progress again in Duo by using this German Memrise course that includes the words in all the https://www.memrise.com/course/335725/comprehensive-german-duolingo-vocabulary/. I'm not sure if similar courses are available for other languages. Memrise does a much better job for teaching words and Duo is better for grammer and sentences.