If you use a 'verb of the senses' (een zingtuiglijk werkwoord, waarmee je iets doet met je zintuigen), you use an adjective where you should normally use an adverb.
Your senses are your eyes, nose, mouth, fingers and ears, so 'verbs of the senses' are: see, smell, taste, feel, hear
For example:
The song sounds beautiful. (not beautifully).
The softness of your jacket feels good. (not well)