"I just do not know what to say."
Translation:Non so proprio cosa dire.
Another general rule, I guess: more often than not, try to keep your adverb after your verb. It takes a little bit of getting used to because it isn't word-for-word translation. In English you could say "I eat quickly" or "I quickly eat"; but in Italian you would never say "rapidamente mangio." "Mangio rapidamente" is correct. "Proprio" is a less obvious adverb, but it still follows the verb. Hope that's helpful somehow.