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be a quack
quack 作名词指‘庸医；冒充内行的人’，为可数名词，常用在 be 动词后作表语。
Don't trust that doctor—he's a quack.
别相信那个医生——他是个庸医。
庸医；冒充医生的人
先理解核心含义，再看它在真实句子里的用法。
庸医；冒充医生的人
an untrained person who pretends to be a doctor and gives medical advice
Don't trust that doctor—he's a quack.
别相信那个医生——他是个庸医。
指冒充医生的人，非鸭叫声。
鸭子的叫声
The vocalisation made by a duck.
假装为医生并提供医疗建议的无执照人员
an untrained person who pretends to be a physician and who dispenses medical advice
鸭子的刺耳叫声
the harsh sound of a duck
The patient soon realized the so-called doctor was a quack who only wanted her money.
病人很快意识到那个所谓的医生只是个只想骗她钱的庸医。
Before taking any herbal cure, check that the practitioner is not a quack.
在服用任何草药疗法之前，要确认这位行医者不是冒牌医生。
My uncle lost his savings to a quack who promised to cure his arthritis.
我叔叔把积蓄都花在了一个承诺能治好他关节炎的庸医身上。
quack 作名词指‘庸医；冒充内行的人’，为可数名词，常用在 be 动词后作表语。
Don't trust that doctor—he's a quack.
别相信那个医生——他是个庸医。
quack 也可作名词表示鸭子的‘嘎嘎叫声’，常用 the quack of + 动物 的结构表示声音来源。
The quack of ducks could be heard from the pond.
从池塘那边能听到鸭子的嘎嘎叫声。
quack 作形容词时只能作定语，常与 doctor、remedy、medicine 等搭配，表示‘冒牌的；骗人的’。
He wasted his money on a quack doctor who promised an instant cure.
他把钱浪费在一个承诺能立即治愈的庸医身上。
quack 作动词是不及物动词，不能直接接宾语，可接副词或介词短语作状语。
The ducks quacked loudly as we approached the lake.
我们走近湖边时，鸭子大声地嘎嘎叫。
Not this
He is quack.
Use this
He is a quack.
quack 作名词表示“庸医、冒牌医生”时是可数名词，单数前需要加不定冠词 a；中文“他是庸医”没有冠词，英语必须用 a quack。
Not this
The duck is quack.
Use this
The duck is quacking.
quack 表示“嘎嘎叫”时是动词，进行时要改成现在分词 quacking；不能把 quack 原形当作表语放在 be 动词后。中文没有词形变化，英语需要用 be + 现在分词。
Not this
I need to see a quack for my cough.
Use this
I need to see a doctor for my cough.
quack 特指没有行医资格却冒充医生的人，是贬义词；如果表达“看普通医生”的意思要用 doctor，把 quack 泛化指任何医生属于词义误用。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
庸医；冒牌医生
She went to a quack doctor and ended up worse.
她去找了一个庸医，结果病情更糟了。
骗人的疗法；江湖偏方
He spent a fortune on quack remedies that did nothing.
他花了很多钱买那些毫无用处的江湖偏方。
假药；冒牌药品
The market was full of quack medicine sold by street vendors.
市场上到处是小贩出售的假药。
骗人的疗法
They claimed their quack cure could heal any disease.
他们声称他们的骗人疗法能治愈任何疾病。
骗人的治疗
The patient refused the quack treatment recommended by the charlatan.
病人拒绝了那个骗子推荐的骗人治疗。
冒牌医生；庸医
He went to a quack doctor who prescribed useless pills.
他去看了一个冒牌医生，对方开了一些没用的药。
骗人的疗法；假药
She spent a fortune on quack remedies that did nothing.
她花了很多钱买假药，但一点用都没有。
如果它看起来像鸭子，走起路来像鸭子，叫起来也像鸭子，那它就是鸭子。
Don't be fooled by the label—if it looks like a duck, walks like a duck, and quacks like a duck, it's a duck.
别被名号骗了——如果它看起来像鸭子，走起路来像鸭子，叫起来也像鸭子，那它就是鸭子。
如果它看起来像鸭子，游起来像鸭子，叫起来也像鸭子，那它很可能就是一只鸭子。
This is a well-known English proverb, often quoted in films and TV shows to describe judging something by its obvious characteristics. The word 'quack' here is the duck's sound, but it also means a fake doctor.
这是一句广为流传的英语谚语，常出现在影视剧中，指根据明显特征来判断事物。这里的 quack 是鸭子的叫声，但它也有“冒牌医生”的意思。
Use quack naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
鸭子叫“嘎嘎”(quack)，不懂医术还“嘎嘎”乱说的医生就是庸医。
语域：informal 常见场景：日常对话、医疗话题讨论 “quack”指“庸医/冒牌医生”，是较口语化的贬称，多用于日常对话或非正式评论；在正式或专业语境中，更常用“charlatan”等词。
这些词都与 quack 押韵，都包含 /æk/ 音：back, lack, pack, black, crack, track。注意：拼写以 qua- 开头的词如 quake /kweɪk/、quality /ˈkwɒləti/ 并不押韵，quay /kiː/ 的发音也很不同。
Dictionary facts from ECDICT, WIKTIONARY.