DuoNinja is a vocabulary learning tool that helps users practice words in a way that their memory actually retains. It uses spaced repetition system (SRS) to show flashcards at optimal intervals for long-term retention.

Each flashcard includes an example sentence and audio so you can see and hear how words are really used. The platform allows you to create your own decks or copy community decks to start studying immediately. You can customize your learning experience by choosing language variants and selecting from hundreds of high-quality voices.

The spaced repetition algorithm shows cards when you're about to forget them, helping words stick with less effort. Users can import their Duolingo vocabulary using a free browser extension that brings words into DuoNinja with just a few clicks.

The system provides full control over your learning process, allowing you to pick specific language variants like Brazilian Portuguese or Mexican Spanish. You can choose from many high-quality voices per language for example sentences and eventually adjust how often you see cards to match your learning pace.

DuoNinja is designed for language learners who want to reinforce their vocabulary acquisition, particularly those already using Duolingo who want to enhance their retention through scientifically-backed spaced repetition techniques.