What is the difference between 변하다 and 변해가다? They mean to change. Are they interchangeable?

Is there any difference between these two expressions?: 그런 거지 뭐 and 그렇지 뭐

What is the difference between: 한 번이라도 최소한 한번 적어도 한번 I understand they all mean "at least once". Are they all the same, used interchangeably?

What is the difference between: 한 번이라도 최소한 한번 적어도 한번 I understand they all mean "at least once". Are they all the same, used interchangeably?

How can I say "retail company" in Korean? As in a company who is retail oriented, thank you :)

Hello, How can I say thick eyebrows and thin eyebrows? I´ve seen the adjectives 굵다 for thick and 가늘다 for thin. Do they fit better in this case than 두껍다 and 얇다 when describing this? Is it correct to say: 굵은 눈썹? 가는 눈썹? I´ve also seen 짙은 눈썹 for thick eyebrows. What is the difference? Please enlighten me!

Which one sounds more natural to Korean people when speaking? -는 것 (거. 게) or -기? For example if someone asks me what I like I know I could use both, but is one more used than the other when speaking? I think I´ve heard 는 거/는 게 more in speaking when watching Korean tv shows 운동하기를 좋아해요 or 운동하는 것을 좋아해요 한국어 공부하기 힘들어요 or 한국어 공부하는 것을 힘들어요

What is the real difference between 을 수 없다/ 못 / 못하다? I've seen a lot of contradicting answers, some say 을 수 없다 is when you can't do something because you are being blocked by something external and not talking about ability and 못 or 못하다 is solely talking about ability or physical state. However, I've also seen it the other way around. I watched a Korean movie where I heard the doctor say 수술할 수 없어요, saying he couldn't perform surgery not because of ability but because the woman had low chances of surviving it and probably would not work. So, this proves 을 수 없다 is by something external and 못 being for ability. Are they interchangeable in some situations? Could you provide some punctual examples?

What is the meaning of 병에 걸리다? I am assuming it means something like getting sick or to catch an illness? I have just never seen it before and came across in in my korean book.