‘So’ and ‘So That’: Coordinating or Subordinating Conjunctions?

Today’s question comes from Matt Mullan, a teacher I met a few years ago at the National Council for Teachers of English convention. He writes, “I struggle to provide students with any explanation for the difference between ‘so’ used as a coordinating conjunction and ‘so’ when it’s really ‘so that’ in disguise.” 

Two Sentences About  Boxes, Pillows, and Cats

This really is a tricky ball of twine to unroll. We’ve even touched on the topic of whether “so” is a coordinating conjunction in episode 424, “Weird Coordinating Conjunctions: ‘Yet,’ ‘For,’ and ‘So.’”

We didn’t pursue the matter too far in that episode, but you better believe we’re going to in this one! To understand Matt’s question, let’s think about two sentences that on the surface are a lot alike. One is “Kim put a pillow on top of the empty box, so no cats would get into it.” The other one is “Kim put a pillow on top of the empty box, so no cats got into it.” The words are exactly the same, except that where one of them has “would get,” the other one has the verb “got.” 

‘So That’ Showing Purpose: A Subordinating Conjunction

Let’s take a closer look at the first sentence, “Kim put a pillow on top of the empty box, so no cats would get into it.” The clause that begins with “so” expresses Kim’s purpose: Kim didn’t want any cats to get into the empty box. Notice that you can replace “so” with the somewhat stuffier phrase “in order that,” and the sentence means the same thing: “Kim put a pillow on top of the empty box, in order that no cats would get into it.” For this reason, it will be convenient to talk about the “so” in this sentence as the “purpose-‘so.’”

Here’s something else you can do with this sentence: You can put the “so” clause first, and the sentence will still be grammatical. In other words, we could also say, “So no cats would get into the empty box, Kim put a pillow on top of it.” 

This ability to come before or after a sentence’s main clause is a dead giveaway that we’re dealing with an adverb clause. An adverb clause is a clause that explains when, where, how, or why the action of the main clause happens. Adverb clauses are a kind of clause known as a subordinate clause, and...

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