"So far" is an idiom meaning;
1. Up to the present moment: So far there's been no word from them.
2. To a limited extent: You can go only so far on five dollars.
In the first sentence you mentioned, the second meaning could have been intended;
... the period was to a limited extent like the present period that some of...
You are correct about using "so" as an adverb to modify adjectives, it should be just before the adjective;
She was so happy that she cried.
She was so beautiful that...
Hope this would help.
As a general pattern, "until" implies something happens immediately after a condition occurs, while "before" does not.
Both "Most children do not start school until they are six years old." and "Most children do not start school before they are six years old." have the same technical meaning. You don't see many children of ages three, four, or five in school. However, the connotations are different. "Until" suggests that children tend to start school as soon as possible once they are six, as though "until they are six years old" is the last hurdle in their way. "Before" also suggests that children start school when they are six, but also suggests that some start at seven or eight, or even never go to school at all.
This pattern is, of course, a fuzzy line. However, it is visible in these two stories about a sprinter:
Do not let your feet start moving before the gun fires. Once the gun fires, give it everything you've got.
Do not let your feet start moving until the gun fires, then give it everything you've got.
In the second sentence the word "until" causes the listener to treat "the gun fires" as a trigger, causing action. This lets us construct the rest of the sentence starting with "then." In the first sentence, with before, it is hard to use "then." The speaker is forced to reintroduce the gun in a second sentence. The following phrasing is awkward
Do not let your feet start moving before the gun fires, then give it everything you've got.
It is probably grammatically correct. However, the first half of the sentence leaves the listener dwelling in the region "before the gun fires," and then the listener has to quickly catch up to the moment for "then give it everything you've got." The sentence formed with "until" brings the listener right up to the moment where the gun fires, so their perspective is more correct for making sense of "give it everything you've got."
Best Answer
First of all I'm not a native english speaker my self, but to me there is a slight distinction which doesn't address whether the action afterwards has completed or not, but more to the point of how the "then situation" is emphasized somehow.
Here are some example which I find has a nice ring with only one of the constructions:
For me the "since then" construction emphasizes the "then situation" more than the other, and gives it greater significance, but this situation is over, and the world has moved on.
Whilst the "from then on" gives more focus to what comes afterwards, the situation has happened, and it has changed something which affects the after situation more.
I can't explain it any better than this, but that is my take on the difference on these very similar phrases. I do believe in a lot of cases they can be interchanged although they might be inflict different importance by the reader regarding the "then situationt" versus the following fact.