If I am going to advise somebody has done some thing wrong in the past in order not to do it again, What shall I say?
If I say:
You must have not done that again.
Can be or not.
If you are going to advise somebody, you can say:
I wouldn't do that again if I were you.
I suggest (that) you don't do that again
I advise you not to do that again.
It's not/It wouldn't be a good idea to do that again.
If you are ordering them, then you can say:
Don't do that again.
You mustn't do that again.
You are not to do that again.