From what I gather Svenska is when it is used as an adjective to describe a noun but use Svansk when it is used as an adverb to describe an adjective or verb.
Here's an example of what I mean:
Den svarta hunden var från Sverige.
(The black dog was from Sweden.)
Den svarta hunden var svenska.
(The black dog was Swedish.)
Han spelade en svensk låt.
(He played a swedish song.)
So I think maybe Svanska is used to describe things you can touch, see, feel, hear. Definites.
Svansk is used to describe things that are more fluid such as songs, languages, and so forth. Maybe that helps.
