Their system is insanely sensitive to mistakes. If I miss I single letter or translate a sentence that isn't the exactly the way they want it, I lose a heart. And this is for English not the language I am learning. I am not trying to learn English, but they don't care, my English must be perfect to use duolingo. For example I had to translate a sentence into English, I only difference between my answer and the correct answer was, "Friday" and "Fridays"