Khan Academy

Duolingo is a language learning application built around short, frequent practice sessions. A lesson is a few minutes of exercises: translate a sentence, match a word to an image, or complete a phrase by tapping word tiles. Progress is measured in a streak of consecutive days rather than in completed units. The design choices are deliberate. Lessons are short enough to fit into a gap in the day, the streak creates an incentive to return, and the difficulty curve is gentle enough that a beginner is not blocked early. A spaced-repetition system decides which vocabulary to revisit, and mistakes are reintroduced rather than dropped. The limitation is depth. The platform builds vocabulary and recognition well but does not, on its own, produce conversational fluency: there is no live conversation partner and limited open-ended production. Learners aiming at fluency typically combine it with speaking practice elsewhere.

Coursera is an online learning platform that hosts courses from universities and companies. A course is a sequence of video lectures, readings, and graded assessments, and completion produces a certificate that can be verified through a public URL. The platform's distinguishing feature is the credential. Courses come from named institutions, and several are assembled into specialisations or professional certificates that represent a longer commitment. Graded work is submitted and evaluated, which is what separates the platform from a video library. The tradeoff is the format. A recorded course cannot adapt to a learner the way a tutor or a cohort programme can, and the completion rates for self-paced online courses are low across the category. The platform also sells degrees, which are a different commitment and price point from the individual courses.

Anki is a flashcard application built on spaced repetition. A card has a front and a back, and after each review the learner rates how difficult the recall was. That rating determines when the card is shown again, so material that is easy is seen rarely and material that is hard is seen often. The scheduling algorithm is the product. Decks are user-created, and the shared deck library means a learner can start from someone else's deck for a standard subject. Because the algorithm is open and the file format is documented, decks can be generated programmatically from other data. The interface is functional rather than polished, and creating good cards is work: a poorly written card produces poor recall no matter how good the schedule is. The application is free on desktop and Android, with a paid iOS client that funds development, and synchronisation runs through a free account.