what is the correct form for good mornings? straight legs or slightly bent? what about the stand?
Either straight or bent legs is OK. There are quite a few different ways of doing good mornings. For the basic method, go here:
Don't know what you mean by the stand. Hopefully Dave Tate's explanation will clarify that for you as well.
He appears to be referring to foot placement, I prefer the stablest foot placement, keeling over with anything over 155lbs is not fun.
the more you bend at the knee the more the hamstrings get involved. The less you bend at the knee the more the erectors are involved. Standard arched back GM involves slight bend at the knees to engage both hamstrings and erectors.
Try and keep your feet placement roughly the same as your squat stance, maybe an inch or 2 wider. A benefit of good mornings is carry over to your squat and deadlift. Work the GMs on the same curve and lever as those lifts and you'll get great results from them.