calm: often implies a contrast with a foregoing or nearby state of agitation or violence: the protests ended, and the streets were ~ again.


tranquil: suggests a deep quietude or composure: the ~ beauty of a formal garden.   （composure平静: 不激动,由于自我控制或性格she maintained her composure during the piano concert她在钢琴音乐会保持了平静）   (中印边境对峙时，双方都要求边境维持tranquil宁静,可以形容人或物)


serene: stresses an unclouded and lofty tranquility: a woman of ~ beauty.     (unclouded不被云遮挡,lofty高的)


placid: suggests lack of excitement or agitation and an equable temper and may imply a degree of complacency: led a very ~ existence.      (equable还形容气候温和不极端)   (complacent自满: 特别是盲目自满，如don't get complacent, set a higher standard不要自满，给自己订立高标准)


peaceful: implies a state of repose often in contrast with or following strife or turmoil: a former firebrand grown ~ in his old age.


calm安静: 强调对比，如之前或周边的激动暴力，tranquil宁静: 深深的安静平静，serene静谧: "高耸入云"的宁静，有文学气，placid平和: 形容脾气温和不轻易激动，peaceful和平: 表示休息安宁，常对比此前的争斗混乱


记忆方法: 1) CTSPP的PP想成两人，CTS想成特充实<==能够平静

               2) 平静的意思是安静而无干扰伤害mean quiet and free from disturbance or harm.CTSPP的PP想成两人，CTS想成特充实<==能够平静