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undercover + noun
undercover 作定语，意为“卧底的、秘密的”，常修饰 agent、operation、officer 等名词。
An undercover agent was sent to investigate the company.
一名卧底特工被派去调查那家公司。
秘密的；暗中进行的；卧底的
先理解核心含义，再看它在真实句子里的用法。
秘密的；暗中进行的；卧底的
undercover
An undercover agent was sent to investigate the company.
一名卧底特工被派去调查那家公司。
秘密进行或发生的
Performed or happening in secret.
受雇从事间谍活动或秘密调查的
Employed or engaged in spying or secret investigation.
The journalist conducted an undercover interview to expose the truth about the company.
这名记者进行了一次秘密采访，以揭露公司的真相。
A police officer was undercover at the night market to catch the thief.
一名警察在夜市卧底，以便抓住小偷。
They prepared an undercover plan to retrieve the documents without being noticed.
他们制定了一个秘密计划，以便在不被人注意的情况下取回文件。
undercover 作定语，意为“卧底的、秘密的”，常修饰 agent、operation、officer 等名词。
An undercover agent was sent to investigate the company.
一名卧底特工被派去调查那家公司。
undercover 作表语时，表示某人处于卧底或秘密工作的状态。
The police officer was undercover during the protest.
那名警察在抗议活动期间是卧底的。
Not this
He is an undercover.
Use this
He is an undercover agent.
Undercover 是形容词，不是名词；汉语的“卧底”可作名词，英语中必须用 undercover agent/officer 等名词短语，不能单独说 an undercover。
Not this
He undercovers in the gang.
Use this
He is undercover in the gang.
汉语里“卧底”可作动词，但 undercover 在英语中是形容词，需用系动词 be 作表语，不能按动词变位。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
卧底特工
The undercover agent infiltrated the criminal gang.
这名卧底特工潜入了犯罪团伙。
秘密行动
Police carried out an undercover operation to catch the drug dealers.
警方开展了一次秘密行动，以抓捕毒贩。
去当卧底；暗中调查
She decided to go undercover to expose the corruption.
她决定去当卧底，揭露腐败。
卧底工作
Undercover work can be extremely dangerous.
卧底工作可能极其危险。
卧底警察
An undercover police officer was assigned to the case.
一名卧底警察被指派负责此案。
秘密调查
The reporter began an undercover investigation into the company.
这名记者开始对该公司进行秘密调查。
卧底；秘密调查
The detective went undercover to gather evidence against the gang.
侦探卧底去收集那个团伙的罪证。
做卧底；秘密工作
She worked undercover for years, and no one discovered her true identity.
她做了多年卧底，谁都没发现她的真实身份。
夜晚在燃烧，夜幕掩护之下。
This lyric is from the Rolling Stones' 1983 hit 'Undercover of the Night.' Here 'undercover' means hidden or concealed by the darkness, not a police officer working secretly.
这句歌词出自滚石乐队1983年的热单《Undercover of the Night》。这里的 undercover 意为‘被夜色隐藏、在暗夜掩护中’，而不是指卧底警察。
Use undercover naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
记住under（在…下）+cover（遮盖）：藏在遮盖之下行动，就是卧底、秘密的。
语域：neutral 常见场景：police work、journalism undercover 通常用于形容警察、记者等以隐藏身份进行的秘密调查或报道，属于中性常用词，在新闻和日常对话中都比较自然。
这些词与 undercover 的词尾 -cover 同韵（重读元音 /ʌ/ 加 /və/）：cover、discover、recover、uncover、lover。hover 是易混词，但在本条目采用的英式读音中读作 /ˈhɒvə/，与 /kʌvə/ 不同韵；部分美式口音中它可能读得接近。
Dictionary facts from ECDICT, WIKTIONARY.