Re: When do babies understand what "no" means, and when do they understand basic discipline? When he pulls my hair, I try to distract or redirect him, but it doesn't always work. I know it's unintentional on his part, but I'm still curious.
Children understand "no" at different ages -- some grasp things earlier than others. You don't need to wait to start explaining the rules, though. At as early as six months, you can begin introducing discipline by verbalizing messages like "we do not hit." Don't expect him to understand or follow those directions, though.
If baby's pulling your hair, start relating the message, "we do not pull hair," and then provide an example of what he can do such as "rub mommy's face." It may take a while, but as baby hears the instructions repeatedly and continues to master physical control over his body, he'll eventually be able to follow the direction.