Spell check errors
I have noticed that, when translating from French to English, Duolingo allows slight spelling mistakes (missing letters, etc.), but rarely (if not never) allows them when translating from English to French. It is extremely common when I add/miss an "e", which, admittedly, is problematic, but can these foreign languages have slight spelling mistakes ignored?
An "e" at the end of an adjective in French, for example, can mean that it is feminine. In German, if you type "dem" instead of "den", it may be just a typo, but it may also be that you don't understand the difference between dative and accusative cases. You are supposed to learn genders and cases well, that is why Duolingo is so unforgiving with these mistakes.
The general rule is: if a mistake could have been made because of some lack of grammar understanding, it is addressed as a mistake and you lose a heart.