Memorization of Vocab and Language Skills Requires more composition in Target Language
Having used Duolingo for several years now, I have noticed that the majority of questions and problems are translating from the target language to the learned language.
This kind of practice, (from merely noticing myself) seems to produce a kind of passive knowledge of the language and vocabulary that often still leaves me puzzled when I try to remember certain words. I believe it would be much more beneficial to include more translation to the target language for developing the ability to think in another language and especially for learning vocabulary by heart.
I do also wonder, though, that as amazing as some of these courses are, it provides no access to actually experiencing the language, and it might actually be a bad idea to try to develop "fluency" in a watered down version of the language with limited connection to real world context. That's a complicate question of language pedagogy, (I think), but I still think it would be much more helpful to push more translating from the target language.
Hope this helps!
Duolingo uses a very good teaching method for beginners and for people, who want to brush up their school knowledge.
In the course "Spanish for English speakers" you are learning the grammar and the pronunciation. You will mostly translate from Spanish to English and the user interface is in English.
In the "reverse tree", the course "English for Spanish speakers", you will mostly translate from English to Spanish. The user interface is in Spanish, and you can start to read (and write) in the Spanish discussion forums.
(Switch off the sound and microphone!)
In the "laddering trees" you can do "Foreign language 2" from "Foreign language 1" and reverse. If you are learning two ore more foreign languages.