HAD BETTER
We use HAD BETTER to give a warning:
You had better not touch my stuff!
Often we use OR ELSE to point out the consequences:
You had better not eat those mushrooms or else you’ll get sick.
In spoken English, we use the contracted form:
You’d better not touch my stuff.
Although it is not technically correct, you will hear people drop the ‘had’ in HAD BETTER:
You better not touch my stuff.