Confused about 서,에서,아서, and 어서.
I got back into learning Korean and I am confused about a few things. In my TTMIK book I just learned in chapter 3 that 그래서 means “therefore” / “so”. I know that there are different ways of saying anything but I today heard that 아서 or that 어서 is more common.. My first question is which of these is more common if they both means what I think they mean. Secondly how do I know whether to use 아서 or 어서? I’m assuming that I look at the latest vowel and decide like I would a conjugation but I am not sure so I wanted to ask just incase.
Secondly I’m confused about 서 and 에서. What’s the difference between both of them. Originally I learned that 서 is added to state a location which is why in a phrase like wait here it’s placed after 여기. What confuses me is I later learned heard that 서 isn’t really used alone and it means the same if not extremely similar as 에서.
Overall just loss when comparing and knowing the difference between the first two and also the last 2. Any help is appreciated