Here's why:
* It modifies a verb, adjective, or another adverb. For example:
* "He was unduly concerned about the exam." (modifies the adjective "concerned")
* "She unduly worried about the project." (modifies the verb "worried")
* "The deadline was set unduly early." (modifies the adverb "early")
* It describes the manner, time, or place of an action or state. "Unduly" suggests something is excessive or inappropriate.
Let me know if you have any more grammar questions!