It's not as simple as saying that the students were misinformed. FANBOY is actually an older formulation than FANBOYS, and there's a complex, and by no means unified history of what words are put in the list.

The big problem here is that FANBOYS is not a homogenous list. "And," and "or" are the pure coordinators; "but," and "nor" are still central coordinators but lack a few of the properties of "and" and "or." The other words in the list have some properties of coordinators and some properties of other parts of speech. "For" and "yet" have a number of coordinator qualities, but also differences from the central coordinators. "So" is even more marginal a member, although it does share many similarities with "yet." Those who go with the FANBOY list are merely drawing the line between coordinator and not-coordinator at a different place.  

Brett Reynolds has an article on the topic that nicely lays out the problems with either formulation: http://www.teslcanadajournal.ca/index.php/tesl/article/viewFile/1092/911


> On Mar 11, 2015, at 9:43 PM, Spruiell, William C <[log in to unmask]> wrote:
> 
> Nathalia,
> 
> One additional trick for approaching this (although Til's "therefore test" is the easiest for most students) is to distinguish coordinating "so" from subordinating "so (that)" -- we don't always say or write the 'that' after the second one, but we can always add it. Using Til's sentences and adding 'that':
> 
> (Works) I went to bed early so that I could get a good night's sleep
> (Ick.)      It is very hot outside today, so that I'm not going to wear a jacket
> 
> This won't work, however, if you have ESL students (since it relies on native-speaker instincts) and also becomes less effective if the sentence is long (since readers get hazy on what the earlier part of the sentence was). 
> 
> Bill Spruiell
> Central Michigan University
> ________________________________________
> From: Assembly for the Teaching of English Grammar [[log in to unmask]] on behalf of Turner, Tildon L. [[log in to unmask]]
> Sent: Thursday, March 12, 2015 12:08 AM
> To: [log in to unmask]
> Subject: Re: Compound Sentences and Coordinating Conjunctions
> 
> Hi Nathalia,
> 
> There is no mystery.  The students were misinformed.  It is always stressful as a teacher to have students who have received inadequate or erroneous instruction of any kind.  The mnemonic is FANBOYS not FANBOY.  The "S" makes all the difference.  "So" is the only coordinating conjunction that indicates material implicature, effectively carrying the meaning of "therefore".  In the college setting in which I teach, I can be forceful about blaming poor previous instruction for difficulties some students have and can dissuade them from continuing to use incorrect grammar.  I appreciate the fact that you may not have such a luxury with younger students.  However, they need to be weaned of the misunderstanding regarding "so".
> Perhaps the students only think of "so" in its adverbial usage such as: "I went to bed early so I could get a good night's sleep", which of course does not require a comma.  They need to become comfortable with a compound like "It is very hot outside today, so I'm not going to wear a jacket", which typically does take a comma.
> 
> 
> All the best,
> 
> Til
> 
> 
> Til Turner
> Languages and Literature
> Northern Virginia Community College
> www.englishiskillingme.com
> Ph: 703.323.3269
> 
> 
> ________________________________________
> From: Assembly for the Teaching of English Grammar [[log in to unmask]] on behalf of Nathalia Hardy [[log in to unmask]]
> Sent: Wednesday, March 11, 2015 8:17 PM
> To: [log in to unmask]
> Subject: Compound Sentences and Coordinating Conjunctions
> 
> I am hoping you could help me solve a bit of a mystery.  Some of my students have been taught that "so" is not a coordinating conjunction and should not be used to join two independent clauses.  Unfortunately, none of them can explain this other than to say "their teacher said so."  These are seventh and eighth grade students.  They have been taught the mnemonic device FANBOY to remember the coordinating conjunctions and are holding firm in their "understanding."  Am I missing something?
> 
> Best,
> 
> Nathalia Hardy
> 
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