This heavily depends on a course, and courses are different between pairs of languages.
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.)
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.)