When to use a comma:
* When "then" introduces a dependent clause: If "then" introduces a clause that cannot stand alone as a complete sentence, a comma is typically required.
* Example: *Then, after a long day, she finally went to bed.*
* For clarity and emphasis: Sometimes, even if "then" doesn't introduce a dependent clause, a comma can be used to separate the introductory phrase from the main clause for clarity and emphasis.
* Example: *Then, with a flourish, she announced the winner.*
When to omit a comma:
* When "then" introduces an independent clause: If "then" introduces a clause that can stand alone as a complete sentence, a comma is not needed.
* Example: *Then she went to bed.*
In short, the decision to use a comma after "then" at the beginning of a sentence depends on whether "then" is introducing a dependent clause or an independent clause.
It's also important to note that style guides might have different preferences regarding commas in certain situations. If you are unsure, consult your chosen style guide for specific guidance.