一旦 | Grammar

Structure

Usage 1: "Once … (then …)"

一旦 + [verb た form] + ら、～
一旦 + [verb dictionary form] + と、～
一旦 + [verb ば form]、～
一旦 + [verb た form] + 以上(は)、～

Usage 2: "Temporarily / for now"

一旦 + [verb]
一旦 + [noun] + を/に/etc.

In Usage 1, 一旦 appears at the beginning of a conditional clause. In Usage 2, 一旦 functions as an adverb modifying the verb/action directly.

Description

一旦 (いったん) has two main usages: (1) expressing that once something happens, a certain consequence follows, and (2) meaning temporarily / for now.

Once … (then …)

The most common usage of 一旦 is in conditional constructions, meaning "once something happens" or "once you do something." It emphasizes that after a certain point is reached, the result is significant or hard to reverse.

It is typically paired with conditional forms like 〜～たら, 〜～と, or 〜～ば.

、べき。

Once you've decided, you should see it through to the end.

、。

Once it starts, you can't stop midway.

、。

Once you lose trust, it's hard to regain it.

This usage carries a nuance of irreversibility or strong consequence – once the threshold is crossed, things change significantly.

Temporarily / For now

一旦 can also mean temporarily or for the time being, indicating that something is done as a provisional or short-term measure before a more permanent action is taken later.

。

Let's take a break here for now.

、。

I'll go home temporarily and then go out again.

In this usage, 一旦 implies that the action is not final – there is an expectation that something else will follow.

Notes

一旦 is used in both formal writing and everyday conversation, though it tends to appear more often in written or semi-formal contexts.
When used in the "once …" sense, it often appears at the beginning of the sentence or clause for emphasis.
Be careful not to confuse this with いちど一度 (いちど), which simply means "one time." 一旦 carries stronger nuance of consequence or temporariness.

Examples

、。

Once you've made a promise, you must keep it.

、。

Once it catches fire, it can't be put out right away.

、。

Let's finish this work for now and then think about what's next.

、。

I'll go home temporarily and change clothes.

、。

Even words you've once learned, you'll forget them if you don't use them.

、。

Let's calm down for now and then discuss it again.

、。

Once he gets angry, he won't forgive you easily.

、。

Please save temporarily and then restart your computer.

、。

The two who had once parted met again.

、。

Let's set this topic aside for now and move on to another matter.

、。

Once you get used to it, this job isn't that difficult.

、。

I went outside temporarily, but noticed I forgot something and came right back.

References

The Adverbial Noun 一旦imabi.org
一旦Bunpro