Here's why:
* A complete sentence needs a subject and a verb. The subject is who or what is performing the action, and the verb is the action itself.
* "Then" is an adverb, not a subject or verb. It tells us about the time of the action, but it doesn't tell us who is doing what.
Example:
* Then I went to the store. (This is a complete sentence because it has the subject "I" and the verb "went".)
* Then. (This is not a complete sentence because it lacks a subject and verb.)
To make a sentence starting with "then" complete, you need to add a subject and verb:
* Then I went to the store.
* Then the sun began to set.
* Then the music started playing.