As a noun
一会儿 means a little while, a short time; or in a moment, presently.
你等我一会儿，好吗？(Nǐ děng wǒ yīhuìr, hǎo ma?) Will you wait for me for a while?
没一会儿，妈妈就回来了。(Méi yīhuìr, māma jiù huíláile.) After a while, my mother came back.
不一会儿饭就做好了。(Bù yīhuìr fàn jiù zuò hǎole.) The meal was ready in a short time.
一会儿我要去学校开会。(Yīhuìr wǒ yào qù xuéxiào kāihuì.) I’m going to the school for a meeting in a while.
As an adverb
一会儿 + Verb/Adjective/Phrases + 一会儿 + Verb/Adjective/Phrases
一会儿 means “now…now…”, “one moment… the next…”, which indicates different situations happen repeatedly.
这孩子一会儿哭，一会儿笑，不知道怎么了。(Zhè háizi yīhuìr kū, yīhuìr xiào, bù zhīdào zěnme le.) The child cried for a while and laughed for a time, not knowing what was wrong.
今天的天气一会儿晴，一会儿阴。(Jīntiān de tiānqì yīhuìr qíng, yīhuìr yīn.) Today’s weather is sunny for a bit and cloudy for a bit.
他一会儿说去旅游，一会儿说要回老家，好像还没决定。(Tā yīhuìr shuō qù lǚyóu, yīhuìr shuō yào huílǎojiā, hǎoxiàng hái méi juédìng.) One moment he said to travel, and the next, he said that he was going back to his hometown as if he hadn’t decided yet.
As a classifier
一下 means one time or once, which is often used after the verb, indicating doing something once or attempting to do something.
不好意思打扰一下，我能问一个问题吗？(Bù hǎo yìsi dǎrǎo yīxià, wǒ néng wèn yīgè wèntí ma?) Excuse me for interrupting. Can I ask a question?
你尝一下这个菜吧。(Nǐ cháng yīxià zhège cài ba.) Please try this dish.
这个游戏很有意思，你们也试一下。(Zhège yóuxì hěn yǒuyìsi, nǐmen yě shì yīxià.) This game is very interesting, you guys try it too.
As an adverb
一下 means in a short while, all at once, or all of a sudden, indicating the time is short or things happen suddenly. In this case, we can also use 一下子.
秋天到了，天气一下就凉了下来。(Qiūtiān dàole, tiānqì yīxià jiù liángle xiàlái.) Autumn is here, and the weather suddenly cools down.
他很厉害，各种难题都能一下子解决。(Tā hěn lìhài, gè zhǒng nántí dōu néng yīxiàzi jiějué.) He is very powerful, and he can solve all kinds of problems at once.
她一下就明白我的意思了。(Tā yīxià jiù míngbái wǒ de yìsile.) She understood what I meant at once.
Note that 一下子 can be used before the verb or at the beginning of the sentence.
听到这个消息，一下子他的脸色就变了。(Tīng dào zhège xiāoxi, yīxiàzi tā de liǎnsè jiù biànle.) Hearing this news, his expression suddenly changed.
教室一下子安静了下来。(Jiàoshì yīxiàzi ānjìngle xiàlái.) The classroom fell silent all at once.
下了一夜大雨，一下子树叶就都掉光了。(Xiàle yīyè dàyǔ, yīxiàzi shùyè jiù dōu diào guāngle.) It rained all night heavily, and all the leaves fell out all at once.
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