The program works its way through the usual topics such as greetings, food, animals, jobs, family, travel, hobbies, clothing, colors and so on. The format engages students by incorporating aspects of gaming including losing lives and earning rewards. Duolingo includes frequent review of the concepts and vocabulary covered and provides immediate feedback as students progress through the levels.
Of course an online course does not replace a teacher or a responsive classroom context, nor does it offer the authentic communicative opportunities that are neccessary to achieve fluency. However, you might find it a useful adjunct to your teaching program or a fun way of reinforcing knowledge and practicing skills.
Another interesting area you may like to explore is the Duolingo blog. Each week blog readers are challenged to provide definitons for 'tricky words' which tend to defy literal translation. Native speakers contribute translations on the Duloingo facebook page and the most popular translation is posted on the official blog. The spanish word 'empalagar' which is shown below was the most popular of the 365 responses posted online.

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