Rules for a left-turn

There are some discussions on the rules for a left-turn, see a previous post entitled >. The discussion point is whether we should wait behind the white line or we are supposed to go over the white line and wait in the intersection for a left-turn. What I understand are as follows:

a) If there is a specific light controlling the left-turn, then we should wait behind the white line for the left-turn-light to turn green.
b) If there is only one light controlling both go-through and left-turn traffic, then we should go over the white line and wait in the intersection for a safe left-turn, when the light is green. Since we have already passed the white line, we can complete this intended left-turn even after the light turns to yellow or red.

Correct me if I am wrong.