Beware the advice of those who are too ready to say it doesn’t matter, particularly in matters of language. Take, for example, this unassuming sentence: My brother called the tenant so I didn’t have to. What does this actually mean? Did my brother realize how busy I was and decide to call the tenant to help me out? Or did my brother just happen to call the tenant, and now that he did, I don’t have to? The first is a question of purpose, and the second a question of result. No small difference.
What’s starting all the trouble here is the little word so. This linguistic bugbear was first an adverb (I was so thankful), but it grew to serve as an adjective (he said it was so), a pronoun (it took a day or so), a noun (the musical note between fa and la), and a conjunction. Our concern here is with the last of these, the conjunction, and the trouble begins to mount when we remember that there are two classes of conjunctions: coordinating and subordinating. A coordinating conjunction connects two independent clauses (clauses with equal grammatical standing or self-sufficiency), and a subordinating conjunction subordinates, or lowers the standing of, one clause to another. When a clause is subordinate, its meaning is complete only in the light of the independent clause to which it is connected: it has lost its own self-determination.
It’s important to remember (and I, for one, wish I had learned this much earlier than I did) that when conjunctions join clauses, they signal the logical connection of the thoughts in each clause; they tell the reader how to relate one idea to another. If I say, for example, that I wanted to go for a walk, but it started to rain, the coordinating conjunction but expresses an opposition between the idea of going for a walk and the idea of its beginning to rain. Opposition is a logical connection, and English grammar classifies opposition, along with addition (and), alternation (or), and result (therefore), as a coordinate idea, something which can stand alone independently. If, on the other hand, I said, if it stops raining, I’ll go for a walk, the subordinating conjunction if expresses the condition on which I will go for a walk. Condition is also a logical connection, but grammar classifies this connection, along with a number of others (reason, cause, concession, purpose), as a subordinate idea, one that makes complete sense only in association with another idea, never in and of itself.
So the problem which our original sentence poses is whether so is serving as a coordinating conjunction indicting result, or as a subordinating conjunction indicating purpose. Properly speaking, some traditional grammars maintain that so should be written so that when it functions as a conjunction (which would have pointed here more clearly to the idea of result), but we now often truncate that phrase and muddle things up with the one word so. Be that as it may, how are we now to sort out this question in order to understand the writer’s intent? As a rule, when a coordinating conjunction joins two clauses, it is preceded by a comma. Thus, if the word so in our example was to be read as a coordinating conjunction of result, the sentence would have included a comma: My brother called the tenant, so I didn’t have to. But if the word so was to be read as a subordinating conjunction of purpose, there would be no comma: My brother called the tenant so I didn’t have to. It appears, then, that in the original version, my brother was trying to do me a favor.
Strength lies in the details, but wisdom, alas, lies in knowing when the details matter. So when I cut my finger the other day, and the slogan on the Band-Aid box told me the bandage I was hurriedly reaching for moves with you, so you won’t miss a moment, was that the purpose of the bandage or the result? But would that detail at the moment really matter?