Here's why:
* It describes nouns: "The food was adequate," "We have adequate supplies."
* It can be modified by adverbs: "The project was barely adequate."
However, it can occasionally function as an adverb in specific contexts, like:
* "The project was completed adequately." (Here, it modifies the verb "completed").
It's important to note that the adverbial form of "adequate" is more common in British English than American English.