Here's why:
* It modifies a verb: It describes how or when an action happens. For example: "They originally planned to go to the beach, but the weather changed." ("Originally" modifies the verb "planned").
* It can also modify an adjective or another adverb: "The originally planned trip was cancelled." ("Originally" modifies the adjective "planned").
Let me know if you have any other words you'd like to identify!