Had better not

Had better not is the negative form of had better. This means that when you use it you advise someone not to go somewhere or not to do something, with consequences following if they do it anyway.

"You had better not cheat on your test."

Means that if you cheat on your test, you probably are going to get a really bad grade, or maybe other punishments.

Like had better, had better not can be contracted to 'd better not. As an example: "You'd better not take those shoes."