Check: Answering a question within the character's field of study has a DC of 10 (for really easy questions), 15 (for basic questions), or 20 to 30 (for really tough questions).

Retry: No. The check represents what the character knows, and thinking about a topic a second time doesn't let the character know something the character never learned in the first place.

Special: An untrained Knowledge check is simply an Intelligence check. Without actual training, a character only knows common knowledge.