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While so can be an adverb or a conjunction, as the latter, it can mean 'therefore': see ODO Definition 1. Yet why did ^synonyms of so fail to become conjunctions as well?
^Footnote: 'hence', 'therefore', 'thus', and other 'final or illative conjunctions' are listed here, but my grammar teacher insists that they're formally only adverbs.
The seven coordinating conjunctions (that is to say words that can be used to connect two independent clauses without one becoming subordinate to the other) are for, and, nor, but, or, yet, and so (acronym FANBOYS). Conjunctions like although and because are subordinating conjunctions, in that they make the clause that follows them depend on the rest of the sentence for their meaning.
Conjunctions, whether coordinating or subordinating, can happily be inserted between two clauses with no more punctuation than a comma. Sticklers for punctuation would tell you that this is not true of therefore, which is not a connector between two clauses and which should be preceded by a semicolon or a full stop when it begins a new clause. The word therefore is a conjunctive adverb. You can see that it behaves like an adverb when it occurs in the middle of a clause: "It was cold and I therefore stayed indoors". Here it answers the question why in relation to the verb stayed. You might choose to place therefore before the subject I and it would continue to play the same role, but it cannot replace the conjunction and. You need at least a semicolon: "It was cold; therefore I stayed indoors".
This is my understanding of the difference: When the ‘so’ in a sentence between two independent clauses could otherwise be replaced with ‘so that’ or ‘in order to’ (identifying a closely-linked causal relationship) then it is doing the job of a subordinator: “She creamed the butter with the sugar so (that) the batter would be well-blended and smooth.”
But when the ‘so’ is used between two independent clauses not intending that same (in order to) relationship, it is functioning as a coordinator: “The movie theater was closed, so my sister and I went bowling instead.”
