Since day one, Duolingo has been dedicated to its mission to develop the best education in the world and make it universally available. Yet, it’s also essential that we develop Duolingo as a sustainable business in order to continually deliver on this mission for our learners.

The Monetization team at Duolingo works on balancing the goals of our mission with the needs of our business by developing useful features for our subscription service, Duolingo Plus, that engage and delight our subscribers on their language-learning journey.

This blog post highlights how we developed a new Plus feature for iOS to help users make the most of learning from their mistakes!

Adding value to our premium offering

Over the last few months, our Monetization team embarked on developing a new feature for Duolingo Plus that could drive our next round of subscriber growth.

We started this effort by taking time to listen to our subscribers. How could we create a premium offering that would help them better meet their language learning goals?

Through multiple rounds of in-depth user interviews, a common theme emerged. Learning a language is hard, and mistakes are a part of the learning process. Learners were looking for a more directed way to practice their mistakes and expected Plus to help them with that goal. With this in mind, we began developing our mistakes practice feature.

Defining our mistakes feature: inspiration from inbox zero

Even if learners told us the importance of reviewing mistakes, we knew it could feel like eating your vegetables to practice what you’re struggling with. That’s why we decided to approach this problem Duolingo-style, by focusing on how we could make this feature fun, rewarding, and bite-sized.

After early prototyping, one concept for mistakes practice stood out: an email inbox. Mistakes would be added to an inbox, represented as a floating button on the Duolingo home screen, with a numbered badge showing a real-time count of a learner's mistakes. Learners could tap the button, launch a short practice session to review those mistakes, and “clear” them from the inbox.

Early wireframe prototype of Mistakes Inbox

Learners loved this feature, both for the learning value of reviewing their mistakes and because they now had one centralized place they could go for that review. The badge with the counter also motivates learners to clear their mistakes; nothing feels more satisfying than getting to inbox zero!

While we were thrilled that subscribers liked this feature, we also wanted to ensure that free users didn’t miss out on the learning benefits of practicing mistakes. Working across several Duolingo teams, we made sure all users would have the opportunity to see their past mistakes again by surfacing them in existing Duolingo lessons.

Surfacing mistakes in existing lessons, available for all users

Applying visual design magic: mending a broken heart

With the concept finalized, we moved on to visual design of the feature.

One of the biggest challenges was finding a good visual representation for the concept of “mistakes.” While we internally thought of the feature as an inbox, iconography around mail inboxes didn’t excite our learners.

We also didn’t want to focus too much on the incorrect nature of the mistakes. Learners wanted to be motivated to review their mistakes, rather than feel punished that they answered a question incorrectly.

Early icon explorations for mistakes

Ultimately, we chose the icon of a broken heart. Learners in Duolingo lose a heart when they make a mistake, so using a broken heart as the symbol for past mistakes resonated well with our learners.

In the final feature, learners are able to correct all their mistakes and “mend” their broken heart into a gold heart!

The final feature design using the broken heart icon


We launched this feature as an A/B test on iOS, and it generated strong results for subscription growth and learner engagement, including a 5% increase in total subscription bookings. Every day, our subscribers are getting the chance to review their mistakes and reach inbox zero. We’re excited to keep investing in this feature to continue to improve our subscriber experience. Stay tuned!