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​Since “later,” “after,’ “then,” and “next”
Are one-word adverbs of time, 
Like “subsequently,” “afterward,”
They live by rules sublime.

Conjunctive adverbs joining thoughts
Expressed as clauses, more,
Must bow with every entrance made,
Give clue to function sure.

The comma brought along is clue
Of clause-wide role ahead
To modify the thought to come
While linking that which led.

A sentence here may illustrate
This role so commonly seen: 
“We'll have to let the fruit get ripe;
Indeed, it's still too green.”

But “then” is special like its peers,
The one-word temporal ones:
“They pranced and danced and lost their pants;
Then all fell down at once.”

Among conjunctive adverbs all,
These words when functioning thus
Are not required a comma to show
But strut with status plus.

The regular adverbs similar are,
For different reason though.
No comma  cometh in between
Such adverb and its bro'.