Here's why:
* Adjectives describe nouns (people, places, things). They answer questions like "What kind?" or "Which one?".
* Adverbs modify verbs, adjectives, or other adverbs. They answer questions like "How?", "When?", "Where?", or "To what extent?".
In the sentence "The instructions were clearly written," "clearly" modifies the verb "written" by describing how the instructions were written.
Let me know if you'd like to explore more examples!