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scum on [surface]
这里 scum 指水面上的浮层或浮渣，常与介词 on 连用。
Green scum on the pond made it look unhealthy.
池塘上的绿色浮渣让它看起来不健康。
浮渣；浮垢；浮藻
scums — plural of scum
先理解核心含义，再看它在真实句子里的用法。
浮渣；浮垢；浮藻
A layer of impurities or algae that forms on the surface of a liquid.
Green scum on the pond made it look unhealthy.
池塘上的绿色浮渣让它看起来不健康。
积聚在液体(尤其是熔融金属或水)表面的杂质层
A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
无用之人
worthless people
漂浮在池塘表面的绿色水生植物(如藻类)
A greenish water vegetation (such as algae), usually found floating on the surface of ponds
液体表面的一层杂质或水生植物薄膜
a film of impurities or vegetation that can form on the surface of a liquid
Those people are absolute scum for cheating the elderly.
那些欺骗老年人的家伙简直是渣滓。
He called the company's management scum after they laid off workers without warning.
在管理层毫无预警地裁员后，他称公司管理层为渣滓。
Don't associate with such scum; they'll only bring you trouble.
别跟这样的渣滓来往，他们只会给你带来麻烦。
这里 scum 指水面上的浮层或浮渣，常与介词 on 连用。
Green scum on the pond made it look unhealthy.
池塘上的绿色浮渣让它看起来不健康。
用于骂人或表示极端鄙视，scum 在这种用法中通常不加冠词。
Anyone who betrays a friend is scum.
背叛朋友的人都是渣滓。
指某一群体中最卑劣、最不齿的人，常见习语 the scum of the earth。
The dictator regarded his opponents as the scum of humanity.
那名独裁者把反对者视为人类的渣滓。
Not this
They are scums.
Use this
They are scum.
当 scum 表示“社会渣滓、卑鄙小人”时是不可数名词，不能按可数名词加复数 -s。英语中用 scum 统称这类人或整体人品，所以 They are scum 自然，而 They are scums 不符合用法。
Not this
There is scum in the surface of the pond.
Use this
There is scum on the surface of the pond.
表示“在……表面上”时，介词用 on 而不用 in；scum 是浮在水面上的一层杂质，不是在水面内部。There is scum on the surface of the pond 才是正确表达。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
社会渣滓；人间败类
He called them the scum of the earth.
他称他们为社会渣滓。
池塘浮渣
The pond was covered with green scum.
池塘上覆盖着一层绿色的浮渣。
人类败类
Those criminals are human scum.
那些罪犯是人类败类。
社会渣滓；人渣
He's a liar and a cheat—the scum of the earth.
他是个骗子和撒谎精——社会渣滓。
坏人得势；小人当道
In unstable times, the scum rises to the top.
在不稳定时期，坏人往往会得势。
Use scums naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
记忆：scum 音近 '屎壳'，让人想到液体表面的浮渣或社会渣滓。
语域：informal 常见场景：贬义口语、侮辱性语境 本词用于贬低某人时非常强烈，通常出现在口语中，极少用于正式场合。
All listed words rhyme with scum (/skʌm/). Be careful of words like 'scam' (/skæm/) which look similar but have a different vowel sound.
Dictionary facts from ECDICT, WIKTIONARY.