
Nicholas Isai C. answered 05/15/24
Master's Student in Classics with 3+ years of studying Ancient Greek
It is very difficult to learn katharevousa without first knowing modern Greek, but if you happen to already know or have a decent grasp of modern (or even ancient) Greek, you can find a textbook written entirely in Greek on katharevousa here:
http://e-library.iep.edu.gr/iep/collection/browse/item.html?code=05-00032&tab=01
Hope this helps!