be discreet in one’s words and deeds;mind (/be on) one’s P’s and Q’s;( )speak and act cautiously
? 朋友們都勸他要～，以免再犯錯誤。All his friends advised him to mind his P’s and Q’s lest he should commit more mistakes.
謹言慎行jǐn yán shèn xínɡ
指說話、辦事十分小心謹慎。speak and act cautiously, be prudent, be discreet in word and deed