知而不言，言而不尽zhī ér bù yán , yán ér bù jìn

though one knows,one will not speak or,if one speaks,one will not tell all one knows
❍ 这些干部、农民、秀才、狱吏、商人和钱粮师爷，就是我的可敬爱的先生，我给他们当学生是必须恭谨勤劳和采取同志态度的，否则他们就不理我，～。(《毛泽东选集》 748) These cadres,the peasants,the Xiucai,the jailer,the merchant and the revenue clerk were all my esteemed teachers,and as their pupil I had to be respectful and diligent and comradely in my attitude; otherwise they would have paid no attention to me,and,though they knew,would not have spoken or,if they spoke,would not have told all they knew.