Anki
apps.ankiweb.net
Schedules flashcard reviews for the day before you would have forgotten them
Free account
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duolingo.com
Turns a language into five-minute daily drills you will actually keep doing.
Bite-sized translation, listening and speaking exercises arranged as a path, with a streak counter doing most of the motivational work. It is very good at vocabulary retention and terrible at teaching you to hold a conversation, which is a fair trade for the first few hundred words of a language. Course depth varies enormously — Spanish and French are years of material, smaller languages run out fast.
Questions
A genuinely usable free tier with paid plans for volume or team features. The free tier is ad-supported and stops you after a set number of mistakes until your hearts refill, and no course takes you past roughly upper-intermediate.
Yes. Duolingo sends your input to its own servers to process it. That is fine for public material; for anything containing personal data, use one of the on-device tools instead.
The free tier is ad-supported and stops you after a set number of mistakes until your hearts refill, and no course takes you past roughly upper-intermediate.
Alternatives
Same category or same task, ordered by how closely they overlap.
apps.ankiweb.net
Schedules flashcard reviews for the day before you would have forgotten them
exercism.org
Practice exercises in 80-odd languages with free feedback from a human mentor
freecodecamp.org
Teaches web development by making you write code in the browser from lesson one
khanacademy.org
Explains school maths and science one short video and one exercise at a time
ocw.mit.edu
Publishes the real lecture notes, problem sets and exams from MIT courses
monkeytype.com
A typing test you can tune to the exact words, punctuation and length you want