When do I use 一下子 vs 突然?

Use 一下子 when you want to emphasize that something happened very quickly and was completed in one go, often with a resultative complement (e.g., 一下子就做完). Use 突然 when the key point is that the event was unexpected or surprising, even if the change itself took a little time (e.g., 突然下雨了).

Can 突然 be used to describe a quick action?

Yes, 突然 can describe quick actions, but only if the surprise element is primary. For pure speed or completion, 一下子 is more appropriate.

Is 突然就 correct?

Yes, 突然就 (e.g., 他突然就走了) is a standard and idiomatic combination meaning 'suddenly and unexpectedly'.

Can 一下子 be negated?

一下子 is rarely negated directly. To express 'not all at once', use 没(有)一下子 (e.g., 他没一下子做完). Negating with 不 is uncommon.

Easily confusedHSK 5

一下子 vs 突然 (yíxiàzi vs tūrán): suddenly or all at once

Both 一下子 and 突然 can express suddenness, but they differ in focus. 一下子 emphasizes that an action or change happens quickly and is completed in a single short interval, often with a resultative complement. 突然 emphasizes the unexpected or surprising nature of an event, and can describe both actions and states. Understanding this distinction helps learners choose the right word for rapid completion versus unexpected occurrence.

一下子 and 突然 both convey abruptness but with different nuances. 一下子 stresses that an action or change happens so quickly that it feels like a single instant, often implying completeness ('in one go'). 突然 focuses on the unexpectedness of an event, regardless of how long the change took. For example, '他突然来了' (He came suddenly, unexpectedly) vs '他一下子来了' (He came all of a sudden, in a flash). 突然 can function as both an adverb and a predicate adjective (e.g., 这件事很突然), while 一下子 is only an adverb. Both can collocate with 就 to intensify suddenness, and '突然就' is perfectly standard.

When to use each

一下子yíxiàzi

all at once, suddenly, in a short while

Use 一下子 when you want to emphasize that an action or change happened very quickly and was completed in a single, swift moment. It often pairs with resultative verb complements (e.g., 一下子吃完了 'finished eating all at once') or describes a rapid change of state (e.g., 天一下子黑了 'the sky suddenly turned dark'). It can also mean 'in one go' for an action done without pause.

一下子 often implies a necessary condition or cause leading to a quick result, and it can be used with 就 for emphasis: 一下子就... 'in just one go'. It is less about surprise and more about speed and completeness.

突然tūrán

suddenly, unexpected

Use 突然 when the key point is that an event happens unexpectedly or is surprising, regardless of whether it happens quickly or over a short time. It can modify verbs (突然来了 'came suddenly'), serve as a predicate adjective (太突然了 'too sudden'), or be used as a noun in some fixed expressions. It often pairs with 就 for emphasis: 突然就 'suddenly and unexpectedly'.

突然 can also describe situations without an explicit action: 突然安静下来 'suddenly became quiet' – note this still describes a change of state (from noisy to quiet), not a static situation. It is the more common word for expressing surprise in speech.

At a glance

	一下子	突然
Focus	Rapid completion of an action/change in one go	Unexpectedness of an event or change
Part of speech	Adverb (cannot be predicate)	Adverb and adjective (can be predicate: 很突然)
Negation	Usually not negated directly; use 没(有)一下子	不突然 (not sudden) is common
Common collocations	一下子就... (emphasizing quick completion)	突然就... (emphasizing suddenness)
Can describe a static situation?	No	No (even 突然安静下来 describes a change of state)

Examples

一下子
他一下子把作业写完了。
Tā yíxiàzi bǎ zuòyè xiě wán le.
He finished his homework all at once.
Emphasizes quick completion in one go.
突然
这个消息太突然了。
Zhège xiāoxi tài tūrán le.
This news is too sudden.
Use 突然 as predicate adjective; note the word is 消息, not 新闻.
突然
他突然就哭了。
Tā tūrán jiù kū le.
He suddenly started crying.
突然就 is fully standard in Mandarin.
一下子
灯一下子灭了。
Dēng yíxiàzi miè le.
The light went out all at once.
Rapid change of state completed in an instant.
一下子
问题一下子解决了。
Wèntí yíxiàzi jiějué le.
The problem was solved in one go.
Implies quick resolution.
突然
天气突然变冷了。
Tiānqì tūrán biàn lěng le.
The weather suddenly turned cold.
Focus on unexpected change.

Common mistakes

Using 突然 for a quick action that is not unexpected, e.g., '他一下子吃完了' is correct, but '他突然吃完了' sounds odd unless the speed itself was surprising.
Using 一下子 as a predicate adjective, e.g., '这件事很一下子' is wrong; use 很突然 instead.
Negating 突然 as '没突然' is incorrect; use '不突然' for 'not sudden'.
Assuming 突然就 is ungrammatical – it is perfectly standard and common.

FAQ

When do I use 一下子 vs 突然?: Use 一下子 when you want to emphasize that something happened very quickly and was completed in one go, often with a resultative complement (e.g., 一下子就做完). Use 突然 when the key point is that the event was unexpected or surprising, even if the change itself took a little time (e.g., 突然下雨了).
Can 突然 be used to describe a quick action?: Yes, 突然 can describe quick actions, but only if the surprise element is primary. For pure speed or completion, 一下子 is more appropriate.
Is 突然就 correct?: Yes, 突然就 (e.g., 他突然就走了) is a standard and idiomatic combination meaning 'suddenly and unexpectedly'.
Can 一下子 be negated?: 一下子 is rarely negated directly. To express 'not all at once', use 没(有)一下子 (e.g., 他没一下子做完). Negating with 不 is uncommon.