With a bit of hard work and a bit of help, most people can achieve a good knowledge of everyday Korean vocabulary.
However, pushing on past that level to build up more advanced vocabulary, and being able to use and recall the kind of words you don't hear every day can be much harder.
Many of these harder-to-learn words are 한자어 or 'Sino-Korean words'. These are the words that are built from Chinese characters - 한자 as they are known in Korean.
Before you stop reading - you don't need to know anything about Chinese characters to benefit from this app! The easier levels focus on the more common characters, which you can start to recognize very quickly. For example, you may not know what '學' means, but you'll come to recognise it as '배울 학': the '학' that means "learn"/"study" in words like '학교', '학생', '과학' and so on. Of course if you do know some of the Chinese characters themselves (or if this program inspires you to start learning a few), you'll be able to benefit from this app even more deeply.
This app does not require any special permissions on your device.
Anki deck available at https://ankiweb.net/shared/info/1500632042
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