sadness: is a general term that carries no suggestion of the cause, extent, or exact nature of low spirits: a feeling of ~ marked the farewell dinner.  告别晚餐


depression: suggests a condition in which one feels let down, disheartened, despondent, or enervated: under a doctor's care for serious ~.     (despondent绝望，指无可挽救的深深悲哀和失望implies a deep sadness arising from a conviction of uselessness of further efforts，如despondent over the death of his father父亲死后他陷入绝望)  (enervate使人虚弱疲倦to make someone feel weak and tired，如an enervating disease/climate使人衰弱的疾病/使人感到乏力的气候)    let down和dishearten泛指失去希望或信心to cause to lose hope or confidence，属于基础词，不和其它同义词做比较  depression抑郁症是致命疾病，是这一组中最严重的


melancholy: suggests a mood of sad and serious but not wholly unpleasant pensiveness: old love letters that gave her cause for ~.    pensive愁思, 表示如梦如愁的情绪，如秦观的"自在飞花轻似梦，无边丝语细如愁"


melancholia: applies to a disordered mental state characterized by settled deep depression: fell into a state of ~ after her husband's death.  settle形容安定，指从不安到安定下来  deep形容程度深


dejection: implies a usually passing mood of one who is downcast or dispirited from a natural or logical cause: a struggling actor used to periods of ~.    (downcast低垂失望，特别有屈辱羞愧的意味implies being overcome by shame, mortification, or loss of hope ，如negative reviews left the actors feeling downcast负面评价让演员感到屈辱羞愧）  注意不要写成despirited，因为没有达到预期目标而失望如a dispirited and resigned expression on her face她脸上气馁和无奈的表情  


gloom: applies to either the extreme sadness of the person afflicted by any of these moods or conditions OR the atmosphere or the effect on others created by one so afflicted: a universal ~ engulfed the devastated town.   解释中OR是分界线


sadness悲伤: 泛指精神低落，但未指明原因范围性质，depression抑郁: 表示失望绝望疲惫的严重状况，melancholy哀愁: 表示真心悲伤的但苦中有甜的愁思（如例句中的情书），melancholia抑郁: 指精神失常的状态，严重抑郁，dejection低落: 通常只是暂时的失望气馁，gloom忧郁阴暗:  可以表示某人自己极为悲伤，或表示给人制造的印象氛围


记忆方法: 1）首字母SDMMDG前三个从前往后想成SaD Man，后三个从后往前想成GooD Man悲伤的人好的人<==悲伤     "悲伤是理解世界的门径"，是一种特殊才能，有它的好处。但过度会损害身体，造成各种疾病，因此应该及时抒解心情学会释放


         2）悲伤的意思是不快乐精神低落mean the state of mind of one who is unhappy or low-spirited.