Anki is a free and open source spaced-repetition flashcard program. You can find many premade decks for it for pretty much any language, and you can create your own cards from real-world content you read (“sentence mining”). You can also find free (or pirated) grammar guides and similar content online. These two resources will give you the foundation for making your input comprehensible, but the vast majority of the learning you’ll do will come from simply reading and listening to native content while making as much of an effort to understand it as possible (such as by looking up words in the dictionary using a browser extension like yomichan)
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