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.

  • Perhaps the answer lies in the etymology. All the adverbs you've given are ultimately related to determiners: hence originally meaning 'from this', therefore meaning 'for that', and thus coming from the same root as this, that. – Anonym Feb 23 '15 at 3:38
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    Your grammar teacher is probly not playing with the full deck of parts of speech. Saying something is an adverb is a confession of ignorance. And if so is a conjunction, so are hence, therefore, and thus. There are a lot of subordinate conjunctions, of various classes, all of which are adverbial in one way or another; mostly, they all introduce adverbial clauses. – John Lawler Feb 23 '15 at 4:17

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".

  • Please provide an example using so to coordinate but not subordinate so the OP will understand how this can be. Bonus points for coordinating actual noun phrases like and, but, or, nor can instead of mere clauses, since there can be no question of subordination then. (NOTE: FANBOYS is a myth, you realize.) – tchrist Feb 24 '15 at 5:22
  • @tchrist I certainly wouldn't wish to claim that FANBOYS embodies an immutable truth. People do use therefore as a connector in the same way that they would use so, and consequently therefore is slowly but surely finding its way into the FANBOYS (FANBOTYS?) canon. But as the OP knows, therefore is not in every context a synonym for so, in the sense that therefore can be used as an adverb of manner, whereas so can't (cf "It was cold and I so stayed indoors"). – Lachlan Dominic Feb 24 '15 at 13:12

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