This app was mentioned in 3 comments, with an average of 1.00 upvote
They also have an iOS app and Android app.
I have used this for a couple of years now.
KOREAN (Language)
Dongsa [Verb conjugation]- iOS / Android / Site
Korean Verb conjugation tool. Small, clean, no advertising. Open Source. Also available on F-droid.
https://play.google.com/store/apps/details?id=us.bravender.android.dongsa