Adjectives modify nouns

Adjectives are words which modify (describe) a noun… not verbs or adverbs or other adjectives.

The girl is beautiful.

Beautiful is modifying the noun girl.

Tom Longboat was not a bad runner. (adjective)

It’s easy to identify the adjective in this sentence.

The foundation seems good. (adjective used as adverb)

This descriptive word is a little more difficult. Even though good is usually an adjective, it’s modifying seems (How does the foundation seem? It seems good.) so it’s an adverb. We can’t answer the question “what is the foundation” because we haven’t shaken it around and tested it yet, so we don’t know if it’s good foundation or bad foundation… we’ll find out for sure if the building falls down.

N.B. Sometimes it may look like an adjective is modifying another adjective, as in the case of dark blue or bright yellow, but this is because modern writing has removed the hyphen from a compound adjective. The proper form is dark-blue and bright-yellow. The evolution of English will eventually change the rules so an adjective can modify another adjective, but formal writing standards don’t allow it yet.