be a counterfeit
counterfeit 作名词时是可数名词，指被冒充为真品的伪造物。
This painting is a counterfeit.
这幅画是赝品。
伪造者；造假者；非正品；假货；冒充原件的复制品
A fake or imitation, especially money or documents, made to look like the original and deceive.
非正品；假货
A non-genuine article; a fake.
冒充原件的复制品
a copy that is represented as the original
伪造者；造假者
One who counterfeits; a counterfeiter.
Experts examined the painting and confirmed it was a counterfeit.
专家检查了那幅画，确认是赝品。
用法提示：本句中，“counterfeit”作名词，意为伪造品。
The market is flooded with counterfeits of popular luxury goods.
市场上充斥着流行奢侈品的仿冒品。
用法提示：本句中，“counterfeits”是名词复数，表示仿冒品。
She was shocked to learn that the gold coin she inherited was actually a counterfeit.
她震惊地得知继承的金币实际上是赝品。
用法提示：本句中，“counterfeit”作名词，指伪造的硬币。
counterfeit 作名词时是可数名词，指被冒充为真品的伪造物。
This painting is a counterfeit.
这幅画是赝品。
of 后接被仿冒的原件，表示某件赝品模仿的对象。
They found a counterfeit of the painting.
他们发现了那幅画的赝品。
declare 后可接宾语和名词补足语，表示正式判定某物为赝品。
The expert declared the banknote a counterfeit.
专家宣布这张钞票是假币。
最常用、较口语化，可指任何冒充真品的物件；语气不如 counterfeit 正式。
尤指伪造的货币、文件、签名或艺术品，强调伪造行为本身。
强调模仿真品，但未必意在欺骗，也可能是标明仿制的产品。
较正式，指并非其所声称之物的东西，语气较强。
美式非正式用语，可指假物或骗子，更口语化。
grammar · medium
易错句子He sold many counterfeit in the street market.
正确表达He sold many counterfeits in the street market.
counterfeit 是可数名词，表示“假货/赝品”时复数形式为 counterfeits；many 后要用复数，不能像中文“假货”那样当作不可数名词。
grammar · medium
易错句子Counterfeit is a serious crime.
正确表达Counterfeiting is a serious crime.
表示“伪造行为”时要用动名词 counterfeiting；counterfeit 作名词指具体的“假货”或“伪造者”，不指行为本身。
针对“伪造品；假货”义项。指原创的正版物品，与counterfeit相对；此反义不适用于“伪造者”义项。
针对“伪造品；假货”义项。指真实、非伪造的物品，强调来源可靠；与counterfeit相对。
fake 泛指任何并非真品的东西，口语常用；counterfeit 更强调刻意仿冒真品或货币，常带欺骗和违法意味。
forgery 特指伪造文件、签名、艺术品或钞票，强调仿冒笔迹或作品；counterfeit 多指仿冒货币或商品。
imitation 强调模仿或仿制，不一定有欺骗意图或违法；counterfeit 强调以假乱真、欺骗他人。
……的赝品
The vase was a counterfeit of a Ming dynasty original.
这个花瓶是明代原作的赝品。
识别赝品
The expert detected the counterfeit within seconds.
专家几秒钟就识别出了那件赝品。
把赝品当正品使用
He was caught passing a counterfeit at the pawnshop.
他在当铺把假货当真货使用时被抓住了。
结果是赝品
The rare stamp turned out to be a counterfeit.
那枚稀有邮票结果被证明是赝品。
制造赝品
The gang was arrested for producing counterfeits of designer handbags.
这个团伙因制造名牌手袋的赝品而被捕。
销售假货
The vendor was selling counterfeits at the market.
那个小贩在市场上卖假货。
counter-（相反；反对）+ -feit（做；制造，源自拉丁语 facere），无后缀
counter-（相反；对抗）+ act（行动）
counter-（对应；相对）+ part（部分）
for-（离开；出去）+ -feit（做；制造）→ 做错事而失去
sur-（超过；过度）+ -feit（做；制造）→ 做得过多
伪造货币是 counterfeit 最常见的语境之一，如假钞。
伪造证件也常使用 counterfeit 描述。
识别 counterfeit 商品或文件时需要验证真伪。
常与 security feature（防伪特征）搭配，用于区分真假。
假冒商品即 counterfeit merchandise，常见于品牌商品领域。
Dictionary facts from ECDICT, WIKTIONARY.