前缀de的含义是 down, away, to undo
de与动词结合时，基本意思是to undo，表示做相反的动作。与名词相结合时，基本意思是 to make down or away，表示降低，剥离，取走
具体示例有：
先观察由de-与词根结合的几个具有代表性的例子。
1.decline [de- = down 下；clin = to bend 弯；-e → " to bend down 弯下去"→]
v. 下倾
ex)The road declines at the turn. 到拐弯处路转下坡了。
2.decrease [de- = down 下；creas = to grow 变得]
v. 减少；减小
ex) Hunger decreases as one eats. 吃东西时饥饿感就减少了。
3.debar [de- = away 外；bar = to shut 关→]
vt. 排除；阻止
ex)He was debarred from club membership because of the scandal. 因为那件丑闻，他被俱乐部拒之门外。
4.detract [de- = away 离；tract = to draw 抽→]
v. 减损；贬低
ex)The ugly frame detracts from the beauty of the picture. 难看的画框影响了这幅的美丽。
最后介绍de-作为动词性前缀与名词结合的例子。
1.declass [de- = to make down 使向下；class n.阶级→]
vt. 使降低或失去社会地位
ex)The baron had some working-class relatives,so he was declassed. 男爵有一些属于劳工阶层的亲属，因此他的社会地位就降低了。
2.degrade [de- = to make down 使向下；grad n.级别 →使某人级别下降]
vt. 使降低，贬黜
ex)The sergeant was degraded to private for disobeying orders. 那中士因为不服从命令被降为士兵。
3.debark [de- = to get away from 从...离开；bark n.船 →]
v. 下（船、飞机等），上岸，登陆
ex)After the passengers debarked in port,the captain debarked his crew. 旅客上岸后，船长打发手下人下船。
详细解释：
de-是个用法较为复杂的前缀。它原是个拉丁介副词，只出现在拉丁或法语借词中。de-依附在词根前，表示dwon(from)或away(from)的意思，有时则表示对词根的强调，有completely 的意思。后来，de-被英语所同化，变成一个相当活跃的派生前缀。它可以加在动词或名词之前生成否定意义的动词。当它与动词结合时，基本意思是to undo,表示“做（与原动词）相反的动作”。当它与名词结合时，基本意思是to make down or away,表示“降低，剥离，取走”等动作。
来源
词根词源字典 www.etymon.cn
