intermittent: stresses breaks in continuity: an ~ correspondence with a distant relative.     (如intermittent bursts of applause一阵阵的掌声，intermittent showers阵雨，如after three hours of intermittent rain, the game was abandoned3个小时断断续续的降雨之后，比赛取消了)


recurrent: stresses repetition and reappearance: the boy suffered from ~ illness.   多用于不好的事物，如本例疾病


periodic: implies recurrence at essentially regular intervals: ~ appearances of a comet.    如periodic table of elements元素周期表，periodic checks定期检查，mailing lists are updated periodically邮件列表定期更新，另外，periodical不是形容词，而是名词期刊


alternate: may apply to two contrasting things appearing repeatedly one after the other OR to every second member of a series: club meetings on ~ Tuesdays.  如alternate layers of fruit and cream水果层和奶油层相间，如her mood alternated between happiness and despair她一会儿高兴一会儿绝望，如the alternation of day and night日夜交替(sense 1)，如John has to work on alternate Sundays约翰每隔一周的星期日得上班，以及本例是sense 2


intermittent断续间歇: 表示经常，但并未连续，recurrent反复: 强调重复重现，periodic周期的: 指重复的间隔有规律，alternate交替、每隔: 指两个事物交替出现发生，或某事物每隔一次出现发生


记忆方法: 1）首字母IRPA排序成PAIR一对<==断续

         2）断续的意思是间歇发生mean occuring or appearing in interrupted sequence.