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

A teacher wanted to know how to explain the difference between "so" and "so that." We have the answer, and sometimes it depends on whether you're writing about a purpose or a result.

By
Neal Whitman, read by Mignon Fogarty,
Episode #683

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

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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 subordinate clauses always begin with a subordinating conjunction. One of the best-known subordinating conjunctions is “because,” so let’s see how this flexible ordering works with a “because” clause. The sentence “No cats got into the empty box, because Kim put a pillow on top of it” could also be phrased, “Because Kim put a pillow on top of the empty box, no cats got into it.”

Another subordinating conjunction is “in order that,” which we were just talking about a minute ago. (That’s right, subordinating conjunctions can consist of sequences of words in addition to individual words.) We know this because we can put a clause beginning with “in order that” at the beginning of a sentence, just like we did with a clause beginning with “because”: “In order that no cats would get into the empty box, Kim put a pillow on top of it.” The fact that the clause “so no cats would get into it” is also able to come at the beginning of a sentence is evidence that it, too, is an adverb clause, and that purpose-“so” is a subordinating conjunction.

Another reason to believe that purpose-“so” is a subordinating conjunction is that it can be replaced by “so that,” which is definitely a subordinating conjunction. We know this the same way as we do for “because” and “in order that”: We can put a “so that” clause before or after the main clause. We can say either “Kim put a pillow on top of the empty box, so that no cats would get into it,” or “So that no cats would get into the empty box, Kim put a pillow on top of it.” 

At this point, it makes sense to conclude not only that purpose-“so” truly is a subordinating conjunction, but also that it and “so that” are actually the same subordinating conjunction. This is what Matt meant when he said that “so” is sometimes “so that” in disguise. Why does it sometimes come along with a “that” and sometimes not? Well, it’s just one of the facts about English that we can sometimes omit the word “that.” For more on that topic, check out episode 601, which we’ll link to in the show notes. 

Here’s what we have so far: Purpose-“so” is a subordinating conjunction, and can be thought of as a “so that” with the word “that” omitted. The clause it introduces can come before or after the main clause in a sentence. Typically, this clause will use a modal auxiliary verb, such as “may,” “might,” or “would,” since these verbs are good for talking about situations that are not true yet, such as purposes. 

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