I think it would be useful to decide wether to add new words to the Flashcard exercise or not.
Everytime i use the flashcards new words are added (3-4 words). I have been using this feature daily (several times) for the last 2 months and now I find it difficult to remember old words, so adding new words every time does not really help with that.
I would rather have the option whether I want new words to be added (probably once I have mastered the current ones) or just review the ones I already have but are not mastered yet without adding new ones.
Of course, this is just a constructive feedback based in my experience so far.