$ – Choose from already-uploaded texts or upload your own that you want to study. You can try the website for free but will need to subscribe to make good use of the service. Click on words in a text to bring up translations or add your own. Your new words can then be studied as flashcards and cloze/multiple-choice exercises.
Anki flashcards app
Download the Anki digital SRS (spaced repetition system) flashcard app (free for Windows, Mac, Linux, and Android devices; available for sale on Apple’s App Store for iOS). Highly recommended software for studying flashcards. You can make your own flashcards or find supplementary Anki decks for Lingualism materials at the bottom of this page. Easy to use out of the box, and highly customizable under the hood.