This post is probably too late but here goes.
Defensive posture is penalized. (bent over, illegal grips etc.).
Stepping off the mat, or being in the red zone too long is penalized.
Joint locks can only attack the elbow.
You really should take a look at the rules on the IJF website.
www.ijf.org
Penalties add up fast in judo. Just 4 minor penalties will end the match.