Here's why:
* It modifies a verb: It tells us *how* something happens.
* Example: "The rain came unexpectedly." (It tells us how the rain came.)
* It can also modify an adjective or another adverb:
* Example: "The party was unexpectedly lively." (Modifies the adjective "lively")
* Example: "The train arrived unexpectedly late." (Modifies the adverb "late")
Let me know if you have any other words you'd like to identify the part of speech for!