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After choosing a language, which level you'd like to start on is chosen. Seeing as I've never spoken French before I chose Basics.
Lessons are numbered and consist of a series of activities that engage the learner rather than lecture. In the top right hand corner there's a yellow button indicating the option to "Test Out." If you think you're good enough at it then go ahead and skip the entire section by testing out of it.
The activities look a little something like the picture on the left. This particular activity is multiple choice but other activities include recording yourself saying the words, spelling, and translating sentences. In the right hand corner there are four hearts. If you lose all your hearts then you must start the lesson over. When a level is completed you gain XP points and you're progress is charted on weekly chart.
You can change the language you're studying at anytime. You can even learn multiple at one time.
Duolingo is perfect for foreign language classes. Teacher's can easily assign one lesson over the weekend along with a worksheet or even use it in the classroom as an activity. It's easy, it's interactive, and it's free!