calm: often implies a contrast with a foregoing or nearby state of agitation or violence: the protests ended and the street 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.


placid: suggests lack of excitement or agitation and an equable temper and may imply a degree of complacency: led a very ~ existence.       (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(unclouded不被云遮挡,lofty高的)，有文学气，placid平和: 形容脾气温和(equable还形容天气温和不极端)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）平静的意思是安静没有干扰伤害mean quiet and free from disturbance or harm.