"You better not do this."
Translation:Lepiej tego nie rób.
Both are the starred answers, actually... geez, the number of such comments, clearly indicating some bugs in the system, is growing and that's really worrying :/
In correct English one would say 'You'd better not do this.' 'You better ...' is wrong as the modal verb is not expressed properly. The incorrect form has arisen because language is learned by being heard, and sometimes the d of You'd is missed.