Then is not a conjunction

We all know our conjunctions, right? I'm going to go ahead and skip over what a conjunction actually does (the function of the conjunction, if you will) for today, but here's a list of acceptable conjunctions:

For
And
Not
But
Or
Yet
So


We were taught the acronym "fan boys" when I was in middle school. Yes, it's dumb. It's a mnemonic. What do you expect?

Anyway, you'll notice the word "then" isn't on this list. That's because it's not a conjunction. Attempting to join two independent clauses with a comma and the word "then" is a comma splice. Observe:

I made cookies, and I went for a walk.


This is acceptable. Either sentence could stand on its own, and they are appropriately joined by a comma and the conjunction "and."

I made cookies, then I went for a walk.


This is a comma splice. "Then" is not a conjunction, so it can't be used to join two complete sentences. There are so many easy ways to correct this problem:

I made cookies. Then I went for a walk.
I made cookies; then I went for a walk.
I made cookies, and I went for a walk.


See how easy that was?