Just helping a friend.
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I like flashcards to learn vocabulary. If you are looking for an app I like Babbel the most. It has a review system where every x days you review the vocab that you already learned to consolidate your knowledge. It really works the best for me.
There are several apps to help with that, like duolingo. But I also like reading familiar books in the new language, like childrens books or fairytales. And watching tv in the new language as well.