Here's why:
* It describes a noun: "The movie was awful." (Describes the movie)
* It can modify another adjective: "It was an awfully long wait." (Modifies "long")
While it's not commonly used as a verb or noun, there are some rare instances:
* Verb (informal): "He awfulled his lines." (Meaning he messed up his lines badly). This is a very informal and slangy usage.
* Noun (slang): "The whole situation was an awful." (Meaning a complete disaster). Again, this is a very informal and slangy usage.
So, while "awful" is mainly an adjective, you should be aware of its other, less common, uses.