Not exactly. Notice that in So they’ll know I sent you, the word so isn’t showing an actual effect. It’s showing an intended effect, or in other words, a purpose. When so shows purpose, it acts like a subordinating conjunction. So which is it, a coordinating conjunction or a subordinating conjunction? One solution would be to say that so is actually a pair of homonyms, one of them a coordinating conjunction and the other a subordinating conjunction. That’s kind of a cop-out, though. On the other hand, it’s cognitively irritating to have so as a coordinator that violates one of our main criteria for coordinators.
Traditional grammar books divide all words into eight nice and neat parts of speech, and our subconscious expectation is that any words that are the same part of speech should be able to do exactly the same kinds of things. The truth is that instead of being like mailboxes in a post office, parts of speech are more like splotches of paint on an artist’s palette. Although it’s easy enough to count how many splotches there are, the edges are blurry, and some tend to run into each other. Nowhere is that clearer than with the small family of words known as coordinating conjunctions.
This podcast was written by Neal Whitman, who has a PhD in linguistics, blogs at LiteralMinded.wordpress.com, and is a regular contributor to the online resource Visual Thesaurus.
Reynolds, Brett. “The Myth of FANBOYS.” 2011. TESL Canada Journal/Revue TESL du Canada 105.29, 104-112. http://www.teslcanadajournal.ca/index.php/tesl/article/view/1092/911