The other day, one of my readers mentioned 忽远忽近 (hū yuǎn hū jìn) in a comment. There are actually loads of four-character phrases you could make by using the same construct. In fact, you can replace 近 (jìn) and 远 (yuǎn) with any two opposite single-character descriptors and form valid phrases, although some maybe more meaningful than others. Following are a few examples to get you started:
忽快忽慢 (hū kuài hū màn) – one moment fast, the next slow
忽前忽后 (hū qián hū hòu) – one moment in front, the next behind
忽高忽低 (hū gāo hū dī) – one moment high, the next low
忽冷忽热 (hū lěng hū rè) – one moment cold, the next warm
忽明忽暗 (hū míng hū àn) – one moment bright, the next dark
Tā de qíngxù hū gāo hū dī, hěn bù wěndìng.
His mood swings between high and low – quite unstable.
Tā duì wǒ hū lěng hū rè.
He is cold to me one moment and affectionate the next moment.
There are a few ways of saying “suddenly” or “all of a sudden” in Chinese.
The adverbs 忽然 (hūrán), 突然 (tūrán), 遽然 (jùrán), 猛然 (měngrán) and 陡然 (dǒurán) all mean suddenly, abruptly or unexpectedly. 忽然 (hūrán) and 突然 (tūrán) are more commonly used in everyday speech. 然 (rán) is a classical word that means “in this manner”.
忽 (hū) means to ignore or to neglect, as in 忽视 (hūshì). You could think of 忽然 as describing something happening suddenly while you have your back turned for a moment.
Wǒmén chūfā de shíhòu hūrán biàntiān le.
When we started out, the weather suddenly changed for the worse.
Tā hūrán gǎibiàn le zhǔyì.
He suddenly changed his mind.
突 (tū) means sticking out or dashing forward, hence unexpectedly or suddenly. 突发事件 (tū fā shìjiàn) is an unexpected incident.
Tā tūrán gǎibiàn le jìhuà.
He suddenly changed his plan.
Wǒ bù zhīdào tā wèishénme tūrán bùlǐ wǒ le.
I don’t know why she suddenly distanced herself from me.
猛 (měng) means violently or vigorously, connoting a sudden change.
Tā měngrán tuī le wǒ yīxià.
He suddenly gave me a push.
陡 (dǒu) means steep or precipitous, again connoting a sudden change.
Chēzi dǒurán tíng xià.
The car suddenly stopped.
遽 (jù) means hastily or being alarmed as when something happens abruptly.
Tā jùrán líkāi le.
He went away hastily.
Colloquially, people also like to use 一下 (yīxià) or 一下子 (yīxiàzi), which means all of a sudden or within a moment.
Méiyǒu xiǎngdào tā yīxiàzi jiù fāpíqi le.
To my surprise, he got angry all of a sudden.
Tā yīxiàzi kū, yīxiàzi xiào.
She alternates between crying and laughing.
Nàxiē jiǎozi yīxiàzi jiù bèi chī guāng le.
Those dumplings were gobbled up in no time at all.