Is sweetly a verb or adjective?

Answer 1

The word 'sweetly' is the adverb form of the adjective 'sweet'.

The word 'sweet' is also a noun form as a word for a food with a high sugar content; a word for an affectionate form of address for a dear or beloved person.

The noun form of the adjective 'sweet' is sweetness.

Answer 2

It is an adverb (answering the question how).