不知不觉

在一个宁静的夜晚，小明坐在书桌前，聚精会神地学习。时间一分一秒地过去，窗外传来阵阵虫鸣，夜风轻轻拂过他的脸颊。不知不觉中，他已经学习了三个小时，疲惫感渐渐袭来，他放下笔，伸了个懒腰，这才发现夜幕已经降临，窗外一片漆黑。

zài yīgè níngjìng de yèwǎn, xiǎomíng zuò zài shūzhuō qián, jù jīng huì shén de xuéxí. shíjiān yī fēn yī miǎo de guòqù, chuāngwài chuán lái zhèn zhèn chóng míng, yèfēng qīng qīng fú guò tā de liǎn jiá. bù zhī bù jué zhōng, tā yǐjīng xuéxí le sān gè xiǎoshí, píbèi gǎn jiànjiàn xí lái, tā fàng xià bǐ, shēn le ge lǎnyāo, cái cèi fāxiàn yèmù yǐjīng jiànglín, chuāngwài yī piàn qīhēi.

On a quiet night, Xiaoming sat at his desk, engrossed in his studies. Time passed by second by second, cicadas chirped outside the window, and the night breeze gently brushed his cheeks. Unknowingly, he had been studying for three hours, and a sense of fatigue gradually came over him. He put down his pen, stretched, and only then did he realize that night had fallen, and the outside was pitch black.

多用于描写事物或行为发生得悄无声息，令人难以察觉。

duō yòng yú miáoxiě shìwù huò xíngwéi fāshēng dé qiāo wú shēng xī, lìng rén nán yǐ chájué.

Often used to describe things or actions that happen quietly and secretly, without being noticed.