1
there is a fly in/on [place]
fly 是可数名词，指苍蝇等双翅目昆虫，复数形式是 flies。
There's a fly in my soup.
我的汤里有只苍蝇。
苍蝇；双翅目昆虫
flys — plural of fly
先理解核心含义，再看它在真实句子里的用法。
苍蝇；双翅目昆虫
A true fly (order Diptera), especially the housefly; any two-winged insect.
There's a fly in my soup.
我的汤里有只苍蝇。
双翅目昆虫；具有两翅(除少数无翅种类外),亦称真蝇。
Any insect of the order Diptera; characterized by having two wings (except for some wingless species), also called true flies.
飞行行为。
An act of flying.
双翅昆虫,善于飞行
two-winged insects characterized by active flight
尤指蝇科昆虫,如家蝇(其他双翅目包括蚊、蠓等)。
Especially, any of the insects of the family Muscidae, such as the common housefly (other families of Diptera include mosquitoes and midges).
A fly landed on the table.
一只苍蝇落在了桌子上。
He unzipped the tent fly and stepped outside.
他拉开帐篷门帘，走了出去。
She noticed the fly of his trousers was undone.
她注意到他裤子的开口没拉上。
fly 是可数名词，指苍蝇等双翅目昆虫，复数形式是 flies。
There's a fly in my soup.
我的汤里有只苍蝇。
此处的 fly 指裤子前襟的拉链开口，与昆虫无关；常用于提醒别人。
Your fly is open.
你的裤子拉链开了。
棒球中 fly 是高飞球，也可说 fly ball。
He hit a fly to left field.
他打出一个左外野高飞球。
健身中 fly 指双臂从两侧向胸前合拢的夹胸动作，常用复数 flies。
I do three sets of flies at the gym.
我在健身房做三组飞鸟夹胸。
Not this
I saw fly on the wall.
Use this
I saw a fly on the wall.
英语中单数可数名词前必须有冠词（如 a/the），汉语没有冠词，因此学习者常漏掉。fly 是可数名词，这里加 a 表示“一只”，句子才完整正确。
Not this
There are many fly in the kitchen.
Use this
There are many flies in the kitchen.
fly 是可数名词，其复数形式是 flies（辅音字母 + y 变 ies）。汉语名词不标记复数，所以学习者容易保留单数形式。用 flies 才能与 many 搭配，表示多只苍蝇。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
拍苍蝇
He tried to swat a fly that was buzzing around his head.
他试图拍打一只在他头上嗡嗡飞的苍蝇。
高飞球
The batter hit a fly ball to deep center field.
击球手打出一个飞向中外野深处的高飞球。
帐篷门帘
The tent fly flapped loudly in the wind.
帐篷门帘在风中哗啦作响。
裤子拉链没拉
He walked around all morning with his fly open.
他整个上午都裤子拉链没拉，四处走动。
飞蝇钓
They went fly fishing on the river last weekend.
他们上周末去河里飞蝇钓鱼。
飞鸟夹胸
He does chest flies with dumbbells to build his pectoral muscles.
他用哑铃做飞鸟夹胸来锻炼胸肌。
美中不足；煞风景的事
The hotel was wonderful, but the slow wifi was a fly in the ointment.
酒店很棒，但网速慢是美中不足。
暗中观察者；墙上的苍蝇
I'd love to be a fly on the wall when they hear the news.
我多想悄悄看看他们听到这个消息时的反应。
纷纷倒下；大批死亡
The workers were dropping like flies in the heat.
工人们热得纷纷倒下。
嘘，苍蝇，别来烦我，我可是有主的人了。
When a fly buzzed around my picnic, I softly sang, 'Shoo fly, don't bother me.'
当一只苍蝇在我野餐时嗡嗡乱飞，我轻声唱道：“嘘，苍蝇，别来烦我。”
蝇王
In Lord of the Flies, the flies swarm around the pig's head on a stick.
在《蝇王》中，苍蝇聚集在棍子上的猪头周围。
Use flys naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
联想：苍蝇会飞(fly)，所以名词fly就是苍蝇；裤门襟、帐篷帘都是可开合的“帘”，也借fly一词表示。
语域：neutral 常见场景：kitchen、park、biology class fly 作名词指苍蝇或双翅目昆虫时，通常为中性词，口语和一般书面语都自然，多用于日常对话、环境卫生或生物介绍中。若指裤子的门襟、帐篷门帘等，则需看具体语境。
All listed words rhyme with fly in the sense of /flaɪ/: their stressed vowel plus following sound is /aɪ/. Watch out for lookalikes ending in -y that are pronounced /ɪ/, such as city and happy; they do not rhyme with fly.
Dictionary facts from ECDICT, WIKTIONARY.