趁虚而入 To take advantage of the situation and enter
Explanation
这个成语指的是趁着对方力量薄弱、防备松懈的时候，抓住时机，快速行动，取得优势。
This idiom refers to seizing the opportunity to act quickly and gain an advantage when the other party is weak and their defenses are lax.
Origin Story
三国时期，魏国大将司马懿率领大军围困蜀国名将诸葛亮。诸葛亮为了迷惑司马懿，故意在军营里摆出要长期坚守的姿态，每天都按时进食，读书看报，司马懿见诸葛亮安然无恙，便认为诸葛亮粮草充足，士气高涨，于是撤军回营。诸葛亮料定司马懿一定会撤军，便派人悄悄地去偷袭司马懿的军营，结果取得了大胜。这就是“趁虚而入”的典故。
During the Three Kingdoms period, Sima Yi, a general of the Wei Kingdom, led his army to besiege Zhuge Liang, a famous general of the Shu Kingdom. In order to deceive Sima Yi, Zhuge Liang deliberately pretended to be prepared to hold out for a long time, eating on time every day, reading books and newspapers. Seeing that Zhuge Liang was safe and sound, Sima Yi believed that Zhuge Liang had sufficient food and high morale, so he withdrew his troops back to camp. Zhuge Liang had predicted that Sima Yi would withdraw, so he sent people to secretly attack Sima Yi's camp, and eventually won a great victory. This is the origin of the idiom “to take advantage of the situation and enter”.
Usage
这个成语常用于比喻抓住时机，快速行动，取得优势。
This idiom is often used to describe seizing the opportunity to act quickly and gain an advantage.
Examples
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敌军内部不和，将军决定趁虚而入，一举攻破敌军。
dí jūn nèi bù bù hé, jiāng jūn jué dìng chèn xū ér rù, yī jǔ gōng pò dí jūn.
The enemy army was in disarray, and the general decided to take advantage of the situation and attack, decisively defeating the enemy.
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创业者需要抓住市场空缺，趁虚而入，才能在竞争中脱颖而出。
chuàng yè zhě xū yào zhuā zhù shì chǎng kōng què, chèn xū ér rù, cái néng zài jìng zhēng zhōng tuō yǐng ér chū.
Entrepreneurs need to seize market gaps and enter early, in order to stand out in the competition.
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学习新知识，要抓住学习的最佳时机，趁虚而入，才能学有所成。
xué xí xīn zhī shì, yào zhuā zhù xué xí de zuì jiā shí jī, chèn xū ér rù, cái néng xué yǒu suǒ chéng.
When learning new knowledge, one must seize the best opportunity to learn and enter early, in order to achieve success.
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我们应该趁虚而入，抓住机会，为公司开拓新的市场。
wǒ men yīng gāi chèn xū ér rù, zhuā zhù jī huì, wèi gōng sī kāi tòu xīn de shì chǎng.
We should take advantage of the situation and seize the opportunity to open up new markets for the company.
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面对对手的疏忽，我们要果断行动，趁虚而入，才能取得胜利。
miàn duì duì shǒu de shū hū, wǒ men yào guǒ duàn xíng dòng, chèn xū ér rù, cái néng qǔ dé shèng lì.
In the face of the opponent's negligence, we must act decisively and seize the opportunity, in order to achieve victory.