How I Use It
I use Duolingo primarily as a homework/extension tool. For students who are more advanced, it allows them to quickly add new vocabulary to their repertoire. For those who are building basics, it is a good way to reinforce vocabulary and sentence structures.
In class, I sometimes use it in pairs or small groups so that students can work together to find patterns in language (gender pronouns, verb endings etc.). We then discuss the patterns they see to build towards grammar mini-lessons.
Duolingo gives students a great idea of their level right from the start. It encourages accuracy in the target language use (which is really important in our middle school language program). Many students can make themselves understood (circumlocution), but once in middle school we expect great accuracy in grammar/gender etc. Duolingo helps to emphasize the importance of this with students.
One of the best things about the app is the ability to set goals in terms of time per day. For students who are serious about learning the target language, they can be ambitious and choose a nice sized chunk of daily practice (25 minutes). The app sends reminders to encourage practice and progress.
I like the variety available in the types of activities students can do. Nevertheless, there is a limit to how much students will want to use this app. Fascination can 'burn out' after a while.