Then or And Then?

The latest Internet fad in defiance of grammar is using "then" as a coordinating conjunction. All the dictionaries seem to have picked it up, but most real publishing houses and journalists thankfully have not. The dictionaries admit that "then" is an adverb or adjective--and despite no rules for being able to use an adverb as a coordinating conjunction--they proceed to say that it is proper to place a comma before "then" and use it as one in the middle of a sentence. They probably consulted AI about it. 

Many teachers are also going along with this craze--except for one who points out very clearly the reasons it cannot be used as one. The gist of her argument is that "then" can be moved around in a sentence--and cannot. 

WRONG: I went to the store, then I ate lunch.  

Consider: I went to the store, I ate lunch then. 

I went to the store, I then ate lunch.

All three of the above sentences (1) contain comma splices, (2) make sense and (3) mean the exact same thing.

Now, let's look at that with a real coordinating conjunction:

I went to the store, and I ate lunch.

I went to the store, I and ate lunch.

I went to the store, I ate and lunch.

I went to the store, I ate lunch and. 

The only one of the above that would be classified as a sentence is the first one. 

My favorite Wall of Shame English Teacher whined that "I ran backward, and then I ran forward" was wrong because it "just didn't sound right to him." No grammar source (read one of the main style guides such as APA, Harvard, or MLA) was his inspiration. Instead, he was teaching his kids what sounded right to him. Sum peepl mite think dis sounds rite. That doesn't make it grammatically correct.

I am familiar with several style guides. None of them would allow you to stick a comma in front of an adverb. 

Apparently, the editors at Oxford, Cambridge, and Webster have all started copying from AI. They all pretty much say the same thing, but Cambridge is most ironic. After stating how to use the adverb "then" as a coordinating conjunction without ever calling it that (because it isn't) it then gives some real world examples ("Examples of then") at the bottom of the page with citations of where they were found. The only time "then" is proceeded by a comma is when it is after an introductory clause. Since all introductory clauses should be followed by a comma--that is what is regulating the use of the comma. You also do not have "and then" after a introductory "if" clause. (Notably, there is no real world use of "then" as a coordinating conjunction in any of Webster's real world examples either.)

I can see how AI would screw this up and come to the conclusion after searching millions of webpages for an answer that you can use a comma before then. After all, how many times do people say, "If X, then Y?" Then is probably the most common word after an introductory phrase. Once one dictionary had their AI determine if this was correct and started it, the rest probably picked it up like wildfire. It just goes to show that even formerly good websites can have bad information on them. With AI taking the place of human writers and editors and the already prevalent problem of one website copying another shamelessly all over the Internet, soon it will no longer have value as an information sharing portal.