Duolingo is doing it amazingly to learn a new language.
I recall having trouble learning English back in high school and then having a breakthrough. The breakthrough was, I figured out how to think in English when trying to debug my errors with the teacher who actually have lived in the USA. Once it clicked, I no longer had to memorise things but predict instead. My English is still not perfect but I can have fluent conversations and almost never need to look up words, I can figure out the meaning of idioms etc.
IMHO, the gamification in Duolingo is not geared towards making you figure out the mechanics behind the language. I think edge case exploration is way to go but the gamification usually revolves around memorising.
E.g. learning Spanish from English on Duolingo is very natural, the course is built the way a baby learns a language, by practice and suggestion, with few small bits of grammar info strewn along the way. According to reports from my friends, learning Spanish from Russian on Duolingo is a completely different experience, all based on memorizing large amounts of rules before any practice even begins, and it works really poorly.
I assume that learning languages on Duolingo is all about building a chain of good courses based on languages you already know by the moment you consider another one. (Only partly joking.)
There is the company called Assimil that has small books based on this concept, where first you are filling in the blanks (not choose from preexisting options) and as the book progresses you have to fill in more and more of the sentence.