Here's why:
* It modifies verbs, adjectives, or other adverbs. It indicates time or sequence.
* "I went to the store, then I came home." (Modifies the verb "came")
* "It was then that I realized my mistake." (Modifies the phrase "I realized")
* "He walked very then." (Modifies the adverb "very")
* It doesn't act as a noun, pronoun, adjective, preposition, conjunction, or interjection.
Let me know if you'd like more examples or have any other questions!