The trick is when you learn it to speak it. It helps me because when you speak it, it will feel strange on your tongue if it's incorrect. That's what I do if I'm not sure and it works 99% of the time.

It depends. If someone say "잘지냈어(요)?" which means "have you been well?" then you can say "응/네, 잘지냈어(요)" which means "yes I have been well". If someone ask you how you are feeling you can say "나는/저는 괜찮아(요)" which means I am Okay or fine.

you say "감사합니다" which means "Thank You". "한국어 열심히하세요" is like saying they are cheering you on in your korean study. Its like saying "work hard on korean!" but in a way to motivate you.

It means that things are set the way they are and will not change. Usually it's used when a person won't change their mind. For example: 1. You will go to your room and study. That is that! 2. You can't try any of my ice cream. That is that. Another expression that is very similar to "that is that" is "set in stone" "That is that" usually has a negative connotation but "set in stone" doesn't always have a negative connotation. For example: 1. I got my paper signed and I'm ready to send it in. Everything is set in stone. 2. I'm sorry. We already picked the cast for the play. It's all set in stone. Hoped I helped^^

Both^^