Fun and engaging, but not flawless.

Duolingo is a gamified language-learning app with more than 500 million downloads, over 40 languages, and rewards for practice: XP, gems and streaks. It’s free, with a paid tier called Super Duolingo. Its users are casual learners, travellers and students.

I reviewed it the way a team would before a redesign: against established usability principles, ranked by what to fix first.

This was a solo project. I used the app as a learner and took screenshots as I went, then checked five areas (home, lessons, profile, settings and pop-ups) against Nielsen’s ten usability heuristics.

Each issue got a severity on a four-step scale, from cosmetic (fix it if there’s time) to catastrophic (it stops people finishing a task), and a proposed fix. I presented it as a design review on April 22, 2025.

• Heuristic 3 · User control and freedom · Big

  In fast matching and multiple-choice rounds, a mistaken tap is checked the moment you make it, with no way to change it.

  Fix: an “undo last choice” button, or let people change an answer before it’s checked.

• Heuristic 5 · Error prevention · Big

  The app is strict about spelling and grammar: a near-miss is marked wrong and costs a heart, and running out brings a refill offer.

  Fix: accept near-correct answers and say “Watch your spelling” instead of penalizing.

• Heuristic 8 · Aesthetic and minimalist design · Big

  Finishing a lesson sets off a run of bright banners, buttons and offers before you’re back on the path.

  Fix: combine XP, gems and “lesson complete” into one clean banner.

• Heuristic 9 · Recognize, diagnose and recover from errors · Small

  A wrong answer gets a red X and the correct answer, but no explanation. “Explain my mistake” opens a paywall.

  Fix: light, in-context feedback without paying for the full explanation.

• Heuristic 1 · Visibility of system status · Small

  When the app first boots, the green screen gives no sign that it’s loading.

  Fix: the loading animation the app already uses in other places.

• Heuristic 10 · Help and documentation · Small

  To see a translation you have to tap the specific word, which is easy to miss.

  Fix: a help button for a quick translation.

This was an evaluation, not a redesign, so nothing shipped. My recommendations: let people undo a choice in fast rounds, soften the spelling check so learning wins over punishment, fold the post-lesson screens into one banner, give clearer in-context feedback on mistakes, and put help inside the lesson.

Most of the issues were small or medium, the kind that wouldn’t take a large change but would have a high impact. What I took away: minor issues hurt usability more than you’d expect, and a heuristic evaluation is a fast way to find them early.

6

Issues across home, lessons, profile, settings and pop-ups.

3

Big issues, the ones to fix first.

0

Catastrophic. None stopped me finishing a lesson.