一下子【yíxiàzi】

一下子 says something happened all at once – suddenly, or in a single go. It goes before the verb, and since it describes an event that has happened, 了 usually follows. 就 often comes in between.

It can stress how much happened at once, as well as how fast.

As the second example shows, it also works with a negative or a potential form: you can't do it in one go.

一下子 or 一下

一下 goes after the verb and softens it: "have a quick look", "wait a moment". 一下子 goes before the verb and means "all at once". Swapping them changes the sentence.

• × 你看一下子 → 你看一下
• × 他明白一下子了 → 他一下子就明白了

突然 also means "suddenly" but only that; 一下子 carries the sense of everything changing in a single moment.