Yohan Yukiya Seseㆍ사요한・謝雪矢@yuuana There is a “learn the letters” section in Duo. It will concentrate on that; although you'll encounter different dialects when it comes to pronunciations.
It's how I learned to read the Korean letters. I ignored the pronunciation, since #DuoLingo didn't bother to explain the rules. Anyway, that's all there is about that app.
The popular online ones (and I think they have an app too) are the following (in no particular order):
* https://www.90daykorean.com
* https://talktomeinkorean.com
They are native Koreans and understands the challenges well, so they explain in detail and explain the rules. ^_^
At least so far, with what's available for free (and their podcasts and videos).
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