Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* Adjectives describe nouns or pronouns. They tell us what kind, how many, or which one.
In the sentence "The house was isolated in the woods," "isolated" describes the noun "house," telling us its location. Therefore, it is an adjective.
Example of "isolated" used as an adjective:
* The *isolated* cabin stood on a hill overlooking the valley.
Example of "isolated" used as an adverb:
* The hiker felt *isolated* in the wilderness. (Here, "isolated" modifies the verb "felt" and describes the degree of the feeling)