"It is something important."
Translation:To coś ważnego.
Why is coś nominative but ważnego genitive? Is the word "important" not being used as an adjective for "something"?
"Coś" generally connects with adjectives in Genitive. Even if it can seem weird, it's just like that. On the other hand, "ktoś" doesn't work in such a way so the form of an adjective always matches the form of "ktoś".