Expressing "all at once" with "yixiazi"

一下子 (yīxiàzi) can be used as an adverb meaning "all at once." It has to come at the beginning of the sentence.

Used as "All at once"

这里的"一下子"表示"一次性"，谓语通常包含数量短语或者跟数量有关的宾语，表示短时间内数量很多或者程度很高。

Subj. + 一下子 + Predicate + 了

他一下吃了三碗米饭。Tā yīxià chī le sān wǎn mǐfàn.He ate three bowls of rice at all once.
你们怎么一下买了这么多吃的？Nǐmen zěnme yīxià mǎi le zhème duō chī de?Why do you buy so much food at all once?
她上个月一下瘦了十斤。One "jin" = 500gTā shàng gè yuè yīxià shòu le shí jīn.She lost ten jin the first last month at all once.

一下子 (yīxiàzi) is a very informal expression to describe how fast things happened for . In colloquial Chinese, "一下" is also used.

天气一下变冷了。Tiānqì yīxià biàn lěng le.The weather is suddenly getting cold.
她下楼的时候 一下子 摔倒了。Tā xiàlóu de shíhou yīxiàzi shuāidǎo le.She suddenly fell when she went downstairs.
我们说话的时候，她 一下子 哭了起来。Wǒmen shuōhuà de shíhou, tā yīxiàzi kū le qǐlai.She burst into tears when we were talking.
那个高高的男人 一下子 抢走了我的包。Nàge gāogāo de nánrén yīxiàzi qiǎngzǒu le wǒ de bāo.That tall guy took my bag out of nowhere.

Subj. + 一下子 + 就 + Verb + 了

听他说完，我 一下子 就明白了。Tīng tā shuō wán, wǒ yīxiàzi jiù míngbai le.After hearing him out, I understood everything all at once.
他是最高的，我一下就看到他了。Tā shì zuì gāo de, wǒ yīxià jiù kàndào tā le.He is the tallest. I spotted him right away.
拍完那部电影以后，他 一下子 就出名了。Pāi wán nà bù diànyǐng yǐhòu, tā yīxiàzi jiù chūmíng le.He became very famous soon after that movie.
看到那张照片，我 一下子 就想起来了。It all flashed back to me when I saw that picture.