Tom bought some flowers that smelled nice.
Why is nice considered an adjective in this sentence?
Is nice not modifying smelled and therefore an adverb?
Smell is a linking verb in this sentence. It links the subject "flowers" to the complement "nice".
This clause tells you about the flowers. It doesn't tell you about how Tom smelled the flowers. So "nice" is an adjective.
You can change "smelled" to another linking verb "were"
Tom bought some flowers that were nice.
smell is one of the sense verbs (along with feel, sound, look, taste, appear, seem) and we use adjectives rather than adverbs with them.
Tom bought some flowers that smelled
nicely.
sounds off and feels like the flowers themselves are sentient and capable of smelling.
Why is nice considered an adjective in this sentence?
It actually is. The flowers smelled nice. They were nice.