Duolingo — the product perspective

Duolingo features a combination of great design and pioneering technology to create a beloved learning experience. In this article, I’d like to take a different approach and uncover hidden science and mechanics that make Duolingo the most popular app for learning languages.

Let’s deconstruct the app by the following categories:

  1. Curriculum and exercises (the heart of language learning app).
  2. Retention features.
  3. Social features.
  4. Monetization.


Duolingo claims to teach spoken language and introduce grammar implicitly with some hints and explanations as per Communicative approach.

At the core of curriculum lies the student model, a computer representation of how well you know the language. The model keeps track of each word you have learned and some properties associated with it to help you pick the “weakest” words for practice. The key component of the model is state-of-the-art Spaced Repetition algorithm that uses machine learning to make each lesson and practice session personalized.

Duolingo curriculum — Progress Tree

Given such knowledge model defined by single words forces Duolingo to introduce new material word by word. You will only see one new word per sentence/exercise. Pedagogically, it hinders the ability to present new material. For example, you learn a simple phrase like “What is your name?” only somewhere in the middle of the Progress Tree. This approach is quite different from the way most ESL materials are structured, where concepts are introduced in chucks, phrases or even whole sentences.

Each unit or Skill is organized around a lexical group, for example — Animals, Food, Professions, etc. and consists of Lessons. Lessons introduce a list of new words, usually, within a context (a sentence consisting of words you already know and a new word). Each word must have a minimum of three example sentences that creates variety, for cases when you want to practice the word, you will always see it combine with different words.

Common misconception is that Duolingo generates example sentences automatically. It recruits volunteers via Incubator where words and example sentences are added for each language. Some words don’t have many example sentences and users complain about their shortage. Duolingo, however, uses some NLP techniques adopted from Apertium project to parse theses sentences and provide corrective feedback.

Structuring you curriculum by lexical groups might interfere with speed of memorization. Instead, many modern textbooks organize curriculum around situations or topics from real life. In essence, it’s much easier for humans to remember “Cow blows a whistle”, then “cow, chicken, duck”, or even better than “whistle, blow, cow” while Duolingo makes you memorize 20 new animals at the same time, instead of introducing them in naturally occurring settings.

A typical course on Duolingo has 60 units — skills, and 180 lessons. Duolingo tries to follow CERF and brake down the sentences they teach in each skill. It introduces about 2000 words in each course and is meant to take you to A2 level of proficiency.

Some other learning theories that Duolingo is based on:

  1. Active recall. The app presents you with translation exercises that are cognitively more challenging than flashcards and multiple-choice. Also, the words are usually presented and assessed within a sentence, which provides even better cognitive exercises, making you remember not only what is the words translation but also how it is used in the sentence.
  2. Core vocabulary. Duolingo instructs their volunteers to create content for courses around most-common words lists, most common conversation situations and most common grammar. The sentences in each lessons can only contain new words introduces in this lesson or words from previous lessons, this probably explain why the sentences sometimes are poor in meaning.
  3. Mastery learning. Students have to complete the prerequisites lessons in order to open the next ones.
  4. Multiple-modes of learning. Duolingo introduces listening and speaking exercises, and also uses images to introduces and test words.

Duolingo approaches testing rigorously and assess different aspects of their app, including the curriculum. For example, they test the best order to introduce Skills and order of words in a Lesson.


Duolingo features eight types of exercises and strives to cover all modes of learning, however most of the exercises fall on translation.

1. Translate from target language(L2) to native language(L1) and vice-versa.

The most popular exercises on the platform, which makes the student translate the sentences and phrases. Duolingo uses this exercise to introduce new words (one per exercise), if the students sees a new word in the sentence, it will be highlighted in orange.

L1 to L2 translation serves as a test challenge that Duolingo uses to calculate Learning metric because it assess both, production and recall.

The app tracks how many times a you looked at hints (translation of the word), and even how much time you took to reply. Those factors are included in calculating word’s strength and decay rate (how fast a student is likely to forget the word).

2. Translate, put words in correct order (mobile only).

It is a nice way to balance typing exercises. This exercise is nice because, you can control for specific words by giving more options of this word and making others easier to guess.

3. Choose correct translations

Another translation variation, which in some cases can be quite useless. Often, the student doesn’t need to understand the content of the options because the answer easily inferred by the sentence length.

4. Translate single word

Translation exercises, which targets single words and can be used to teach special cases like gender article in French.

Single word translation — Duolingo

5. Translate the pairs (mobile only)

Unique to Duolingo, this exercise probably serves as a refresher for the words that have a high failure rate, as it is easy to guess the answers.

Tap the pairs translation — Duolingo

6. Fill in the blank (multiple-choice)

Swiss-army knife exercise that helps assess grammar concepts or vocabulary.

Fill in the blank — Duolingo

7. Speaking

The web (Google Chrome) version of the app has impressive accuracy (using Chrome Speech Recognition engine), but mobile versions can accept humming as a correct answer.

Speaking — Duolingo

8. Listening

Classic exercises, which was tailored nicely into Duolingo’s UI. The robotic voice might be scary sometimes, but according to Luis Von Ahn, switching to the native speaker’s voices hurts their retention.

Listening — Duolingo


The app has gamification features such as XP points, Levels, Lingots, Lingot Store, and Fluency score.

The XP and Levels show how hard your work in the app, while proficiency level is displayed by the Fluency Score.

The challenge with XP system is that it’s hard to tie it to the progress in the curriculum. In game design, XP and Levels are used to display user’s progress, but they are also used to unlock new content and features. In Duolingo, the only function of XP and Levels is to compete with your followers in the leaderboard.

The XP system can be improved by introducing rewards or abilities that open up when reaching a certain level. For example, limit features accessibility, such as “Clubs” until a user reaches a certain level.

When completing skills and reaching new levels, you are rewarded with lingots. You can spend them in the Lingot store to purchases bonus skills, take special quizzes, or buy outfits for the owl. Bonus skills have the biggest value, while the owl’s outfits are completely useless because nobody, besides the student, will see them.

Those rewards don’t contribute to student’s ability to “show-off” their progress. Game designers have employed this trick for years letting you customize game environment (such as your castle or village) and avatars. The key point here is that other players/students can see and set their goals around cool items others have.

Coach and Lingot store

Coach and Lingot store


Immersion was Duolingo’s initial approach to monetization where students would translate real-life content in exchange for corrections from native speakers; however, this feature has now been officially shut down. The CEO has mentioned that it was only used by 5% of the audience and this was one of the main reasons to shut it down. The news were not well received by senior users who finished all Skills in the app and used Immersion to practice on real-life content.

New updates

Duolingo is currently working on the update where they are planning to introduce Gems and Customizable User Avatars. Gems are, so called, hard-currency that users purchase with real money in order to progress faster or buy decorative items in the app. In Duolingo’s case, students will use gems to buy special items for their avatar (earrings, glasses, etc.) and refill Health. Health extends Heart mechanics used in tests, where users would be penalized for every mistake they make ( up to 5 mistakes ). The health refills slowly, about 2 hours of waiting to refill one mistake, and users are offered to refill it with gems that can easily be bought with cash.

The students can also earn health by practicing learned words, which is great way to motivate students in the right direction (as a teacher, Duolingo, wants students to practice and repeat what they have learned).

It’s a brave and pioneering move among language learning apps, which generated a lot of controversy both among users and educators. But I sincerely hope they succeed, because then Duolingo would prove that free-to-play monetization is viable beyond gaming industry, which might start a new wave of educational apps.

Social features

Duolingo at the moment has a variety of social features. Some of them serve purely educational purpose and others were borrowed from the game industry to involve students into the experience.

Sentence Discussion

Speculatively, the coolest social feature, loved by many users, is where students can discuss and tutor each other on a particular exercise, sentence or concept.


Reddit like forum where users can up-vote each others answers and to ability to gift lingots for exceptional answers.


The students can follow each other and they will show up in their Activity feed (web only), leaderboard, and Friends tab on mobile. It is inconsistent between devices, yet has some popularity among the users. It seems that Duolingo is cannibalising it with the Clubs.


The actively developed version of “clan” mechanic from game design, where users can create a group and ask other people to join via a code. The club will have their own Activity feed and leaderboard.

It’s hard to say that this is a big improvement over Friends feature as the functionality is still limited and doesn’t have any critical engagement features. Games such as Clash Royal, allure players to join a club by providing special functionality that lets players progress faster and receive bonuses.


  1. Duolingo English Test. Costs 49$ and is indeed seems like a great alternative to IELTS and TOEFL. The science behind the test is impressive, as Duolingo spends almost zero time preparing and assessing the exam — everything is done by the computer. There are some competitors in this area, notably EF English Test, which compared to Duolingo’s resembles classic English assessment and is offered completely for free.
  2. Advertisement on Android. There is one place where a full-page add is shown whenever you complete a practice session or a lesson. I haven’t noticed similar ads on iOS or Web platforms.
  3. Freemium currency. Buy gems to progress faster (details in the Retention section).
  4. Adds-free. Recently, Duolingo added ability to buy adds-free version for approx. $9.99.


Duolingo strength are engineering and design. It’s a unique blend of the latest advances in machine learning and great UI. However, crowdsourced content and certain dogmas in curriculum design might not be the best approach to teaching foreign languages as the company struggles to find a concrete business model.

Note: Things not covered in this article are Incubator, Duolingo for Schools and Bots.

Other articles to consider:

  1. Laura Hale blog post that criticizes translation and lack of grammar.
  2. Kerstin Hammes criticism of vocabulary, translation, robotic and lack of grammar explanations.
  3. Users perspective on Duolingo and Memrise.
  4. https://smartergerman.com/german-language/learn-a-language-online-with-duolingo/