I think some infants start to understand what “no” means at around 6 months, but most won’t stop what they’re doing in response to the word until they’re somewhere between 12 and 18 months. When your baby gets into mischief, calmly tell him no and try to interest him in something else.

They don’t always listen but you need to he persistent when you really mean it so they will learn in a simple way and be consistant

My grandson has just turned one. His parents tell him know, he seems to know what it means as he usually has a hissy fit, but he doesn’t always listen to it