AnkiDroid lets you learn flashcards very efficiently by showing them just before you forget. Flashcards can include images/sounds. It is usable offline and syncs with other Anki apps for Windows/Mac/Linux/Chrome.
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Pro Huge selection
AnkiDroid offers more than 6000 topics to learn from (not all are languages).
Pro Sync with other computers and devices
Synchronize data with Windows/Mac/Linux/iOS and other Android devices.
Pro Displays flash cards based on need
Each time a card is remembered, the user rates how well they remembered it on a scale of 1-3. Tougher words will be shown more frequently, and easier words will be showed less frequently.
Pro Unmatched customizability
You can design your own cards and adjust the spaced repetition algorithm to match your learning preferences.
Pro Access to dialogues
Each language offers access to more than 150 dialogues between native speakers. These can help build a more natural understanding for the learner.
Con Syncing style takes getting used to
Con Dated interface
Con User-generated decks vary widely in quality and accuracy
Con Wonky audio performance
With certain decks, the audio feature in this app won't work.