Here's why:
* Adjectives describe nouns: "Unsure" describes a state of being, often referring to a person's feelings or knowledge. For example, "He was unsure about the answer."
* Verbs express actions or states of being: There's no action associated with "unsure."
* Nouns name things, people, places, or ideas: "Unsure" doesn't name anything concrete.
Let me know if you have any more questions!