Modifiers of manner describe “how" something was done. How words are used in a sentence is essential. Look at these comparisons:
The coffee tastes bad.
He liked the quick service.
The rain was very heavy.
I performed badly.
The staff served quickly.
It was raining heavily.
In the first three, only adjectives are used as modifiers: “bad coffee," “quick service," “heavy rain." In the second three, adjectives are used as adverbs: “badly performed," “quickly served," “heavily raining."
Be very careful while dealing with the “feel verbs": to taste, to smell, to see, to think, etc. These verbs act as adjectives. Compare:
The man sees badly; he needs glasses.----- (His action of seeing is bad)
The coffee tastes bad.----- (The coffee, not the action of tasting, is bad)
The music was very quiet.----- (The music—noun— was quiet)
The music was played quietly.----- (The action of playing was quiet)
He looked so serious.----- (The man was serious)
He looked seriously at the project.----- (The action was serious)