The lessons progress cleverly. Duolingo doesn't begin by providing a boring grammar lesson or even point to ways in which is the target language is different. It simply starts by allowing one to use words. Sometimes I have to translate and type text into English. At other times, the learner must listen and type in the foreign text. Often, one has to translate a sentence by selecting from a number of possible words. Finally, the last type of activity occurs when one must repeat and read into the microphone to check for accuracy.
I always play on my Android tablet, but the results are in the cloud meaning I can continue from any web browser. (It is also available on iOS for those with Apple products.)
Oops, did you see how I said PLAY in that last paragraph? That I guess sums up the beauty of Duolingo's courses. You might wonder how they can keep their service completely free. The link in the first sentence of this entry will take you to their Wikipedia entry. Very clever indeed.
A lot of teaching time, especially when there's necessary repetition, should be taken care of by motivated students and very patient and capable devices.