
Quinn K. answered 10/13/20
Advanced Korean Tutor with 2 years professional/tutoring experience
To add to the great resources already provided, "How to Study Korean" is a very good website made by a Korean teacher who specialized in Korean Grammar for english speakers. The teacher is fanatic about teaching Grammar, and the site has a full document for almost every grammar particle in the Korean language. These write-ups can get a little wordy at times, but upon finishing one you will feel 100% more competent in using any given grammatical particle. While not quite as convenient as a grammar program that allows you to plug in your Korean sentences and it will check on the fly, "How to study Korean" is a perfect resource for studious pupils who want to perfect and become more confident in naturally producing/using Korean grammar particles.
https://www.howtostudykorean.com/
Another good resource that I recommend to all my students would be a Korean dictionary, either a Korean-English dictionary or a native Korean dictionary, depending on your level. Korean dictionaries always have numerous example sentences for each word, giving you a base for how to naturally use any vocabulary in the Korean language. They can be purchased inexpensively on Amazon, and ship all over.