While Duolingo and Busuu both offer free and paid-for premium versions, Babbel is only free for the first week. All three have put a lot of work into their software to try to make the learning process as effective as possible. But are they actually any good? How proficient can people get in another language by using them? Renowned linguist Stephen Krashen, professor emeritus of the University of Southern California, is not very complimentary. "My analyses of their results show that they achieve the same mediocre results as traditional methods do in regular classrooms, and produce the same lack of enthusiasm from students," he says. "We don't acquire language by speaking or writing, even when we get our errors corrected. Rather, the ability to speak and write fluently and accurately is the result of language acquisition via comprehensible input, such as good books, movies and interesting stories."

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