这其实是你不该关心的问题，你的重点应该在多阅读，多体会上。

The Free Dictionary 关于so的用法的说明：

Usage Note: Many critics and grammarians have insisted that so must be followed by that in formal writing when used to introduce a clause giving the reason for or purpose of an action: He stayed so that he could see the second feature. But since many respected writers use so for so that in formal writing, it seems best to consider the issue one of stylistic preference: The store stays open late so (or so that) people who work all day can buy groceries. Both so and so that are acceptably used to introduce clauses that state a result or consequence: The Bay Bridge was still closed, so (or so that) the drive from San Francisco to the Berkeley campus took an hour and a half. · Critics have sometimes objected to the use of so as an intensive meaning "to a great degree or extent," as in We were so relieved to learn that the deadline had been extended. This usage is most common in informal contexts, perhaps because, unlike the neutral very, it presumes that the listener or reader will be sympathetic to the speaker's evaluation of the situation. Thus one would be more apt to say It was so unfair of them not to invite you than to say It was so fortunate that I didn't have to put up with your company. For just this reason, the construction may occasionally be used to good effect in more formal contexts to invite the reader to take the point of view of the speaker or subject: The request seemed to her to be quite reasonable; it was so unfair of the manager to refuse. · New England speakers often use a negative form such as so didn't where others would use the positive so did, as in Sophie ate all her strawberries and so didn't Amelia. Since this usage may confuse a speaker who has not previously encountered it, it is best avoided in writing.

The Free Dictionary：so作为副词的词条下有以下注释：

Usage: In formal English, so is not used as a conjunction, to indicate either purpose (he left by a back door so he couldavoid photographers) or result (the project was abandoned 

so his services were no longer needed). In the former case to or in order to 

should be used instead, and in the latter case and so or and therefore would be more acceptable. The expression so/therefore should not be used。

夸克《大全》

Coordinators identified
13.5 We regard these conjunctions as clearly coordinators: and, or, and but. And
and or are central coordinators, but but differs from them in some respects.
On the gradient between 'pure' coordinators and 'pure' subordinators are for
and so that (meaning in this chapter, unless otherwise stated, 'with the result
that'). There is also a gradient relating coordinators to conjuncts : for example,
yet and so are two words which we shall classify as conjuncts, but which
nevertheless resemble coordinators in some respects. Nor is not a clear case
of a coordinator, since for many speakers it can be preceded by another
coordinator (a potentiality not available to the central cases and, or, and but,
as we shall see). Nor also contains a negative feature, which introduces some
syntactic differences (cf 13.36ff).