1. Check Definition: ANY piece is in "check" if it's unprotected AND attacked
2. Moving into Check: ILLEGAL! Can't move any piece to a square where it would be in check
3. Leaving Piece in Check: ILLEGAL! Can't make a move that leaves ANY friendly piece in check
4. King Rule: King is like any other piece (no special treatment, can be captured)
5. Game End: CHECKMATE if you have no legal moves AND any piece is in check. STALEMATE if you have no legal moves AND no piece is in check.
6. All other piece moves, castling, promotion, pawn moves, are identical to normal chess
Translation: You can move to any square unless the piece would be attacked AND undefended there.