〜てしまう／〜ちゃう: Completion & Regret | Japanese Grammar

Attach しまう to a te-form and you do one of two things that sound like opposites but share a single core: you either finish an action completely, or you regret having done it. 宿題を全部やってしまった can be a satisfied "I got it all done," while 電車に傘を忘れてしまった is an exasperated "I went and left my umbrella on the train." This page shows you how one auxiliary covers both, why the two readings are really the same idea, and how the everyday contractions 〜ちゃう / 〜じゃう work — including the sound split that trips up almost everyone.

The form

Take the te-form of any verb and add しまう. It conjugates like an ordinary godan verb: しまいます (polite), しまった (plain past), しまいました (polite past).

Verb	て-form	しまう
食べる (to eat)	食べて	食べてしまう
やる (to do)	やって	やってしまう
忘れる (to forget)	忘れて	忘れてしまう
読む (to read)	読んで	読んでしまう

Notice 読む → 読んでしまう. Verbs whose te-form ends in で (the ぐ・む・ぶ・ぬ verbs) keep that で before しまう. This で/て split becomes the whole story once we reach the contractions below.

Reading 1 — completion, all the way through

The plain verb しまう means "to put something away, to finish up, to be done with it." Used as an auxiliary, it keeps that flavor: the action is carried through to the end and put away. English often reaches for "up" (eat it all up, use it up, finish it off).

宿題を全部やってしまった。

shukudai o zenbu yatte shimatta

I got all my homework done.

この小説、面白くて一晩で読んでしまった。

kono shōsetsu, omoshirokute hitoban de yonde shimatta

This novel was so good I read the whole thing in one night.

ケーキを一人で全部食べてしまった。

kēki o hitori de zenbu tabete shimatta

I ate the entire cake by myself.

Here しまう adds finality and thoroughness. 全部 ("all") often rides along, reinforcing the "right to the end" sense — but it is しまう, not 全部, that carries the note of sealing the action shut.

Reading 2 — regret, the unintended "oops"

The same しまう, laid over an action you did not mean to do, reads as regret: you ended up doing it, and now it can't be undone. English translates this with "end up …-ing," an exasperated "went and …," or a rueful "unfortunately."

電車に傘を忘れてしまった。

densha ni kasa o wasurete shimatta

I went and left my umbrella on the train.

つい本音を言ってしまった。

tsui honne o itte shimatta

I let my real feelings slip out.

大事なメールを間違えて消してしまった。

daiji na mēru o machigaete keshite shimatta

I accidentally deleted an important email.

ダイエット中なのに、また食べてしまった。

daietto-chū na noni, mata tabete shimatta

I'm on a diet, and yet I ended up eating again.

The adverb つい ("without meaning to") and 間違えて ("by mistake") are natural companions here, but even without them, しまう alone signals that the outcome slipped out of your control.

The shared core: irreversibility

Do not file these as two unrelated meanings to memorize separately. They are one idea. しまう seals the action as done and finished, beyond taking back — and whether that sealing feels good or bad depends entirely on whether you wanted the outcome.

Finish a goal → the seal is satisfying: "done, accomplished."
Make a mistake → the seal is painful: "oops, and now it's irreversible."

That very irreversibility is what English speakers keep missing. Learners are taught "completion" first, then hear 遅刻してしまった and translate it flatly as "I was late (completely)," losing the speaker's cringe entirely. The finality is neutral; the feeling comes from context and intonation.

コップを割ってしまって、朝から最悪だ。

koppu o watte shimatte, asa kara saiaku da

I broke a glass, so my morning's already off to a terrible start.

💡

When you meet 〜てしまった, ask one question: did the speaker want this? If yes, it's proud completion; if no, it's rueful regret. The grammar is identical — only the situation tells you which. A falling, sighing intonation almost always signals the regret reading.

The casual contractions: 〜ちゃう and 〜じゃう

In speech and casual writing, てしまう collapses. This is one of the most common contractions in spoken Japanese, and you will hear it constantly. The split follows the て/で of the te-form:

te-form ends in…	てしまう →	Example
て (食べて, 言って, 忘れて)	ちゃう	食べちゃう, 言っちゃう, 忘れちゃう
で (読んで, 飲んで, 死んで)	じゃう	読んじゃう, 飲んじゃう, 死んじゃう

The rule is mechanical: て + しまう → ちゃう, and で + しまう → じゃう. Voiceless stays voiceless, voiced stays voiced. Past tense follows: ちゃった / じゃった.

もうケーキ、全部食べちゃった。

mō kēki, zenbu tabechatta

I already ate the whole cake.

ビール、一人で全部飲んじゃった。

bīru, hitori de zenbu nonjatta

I drank all the beer by myself.

ごめん、財布、家に忘れちゃった。

gomen, saifu, ie ni wasurechatta

Sorry, I left my wallet at home.

早くしないと、電車、行っちゃうよ。

hayaku shinai to, densha, icchau yo

If you don't hurry, the train's going to leave (on us).

The single most reliable way to spot ちゃう / じゃう is to expand it back: 飲んじゃった unpacks to 飲んでしまった, 忘れちゃった to 忘れてしまった. If you can't run that expansion, you can't be sure of the meaning — so drill it until it's automatic.

💡

There is a hyper-casual further contraction 〜ちまう／〜じまう (やっちまった "I've gone and done it"), which sounds rough, masculine, or old-fashioned (informal / regional). In Kansai you'll instead hear 〜てまう and its past 〜てもうた (regional: Kansai). Recognize them; the standard ちゃう／じゃう is what to actually use.

Register

てしまう is register-neutral: it works in polite speech (食べてしまいました), formal writing, and news reports. The contractions ちゃう / じゃう are strictly (informal) — natural with friends and family, out of place in a business email. If you want the completion or regret nuance in a polite context, use the full 〜てしまいました, not ちゃいました (which is casual-polite at best). See casual speech for where the contractions belong.

How this differs from English

English has no single grammatical marker for "sealed, irreversible completion." It scatters the job across phrasal particles (eat it up, use it up), and across whole idioms for regret (I ended up …-ing, I went and …). Japanese folds all of that into one auxiliary. That is why a learner who reaches only for the dictionary meaning of the main verb — "I ate the cake" for ケーキを食べてしまった — drops the entire emotional and aspectual layer the sentence was built to carry.

It also pairs instructively with 〜ておく: both are te-form auxiliaries about how an action relates to time, but 〜ておく looks forward (do X now, in preparation), while 〜てしまう looks back at an action now sealed shut.

Common mistakes

❌ 飲んちゃった

nonchatta

Incorrect — 飲む's te-form is 飲んで (で), so it must contract to じゃった.

✅ 飲んじゃった

nonjatta

I ended up drinking it. (読む・飲む・遊ぶ・死ぬ → じゃう)

❌ 遅刻してしまった。

chikoku shite shimatta

Under-read as a flat 'I was completely late' — that discards the regret しまった adds.

✅ 遅刻してしまった。

chikoku shite shimatta

I went and showed up late. (rueful 'oops' — that regret is the whole point of しまった)

❌ 宿題を全部やるしまった。

shukudai o zenbu yaru shimatta

Incorrect — しまう attaches to the te-form, never the dictionary form.

✅ 宿題を全部やってしまった。

shukudai o zenbu yatte shimatta

I got all my homework done.

❌ 財布を忘れてしまいたです。

saifu o wasurete shimaita desu

Incorrect — しまう is a godan verb; the polite past is しまいました, not しまいた.

✅ 財布を忘れてしまいました。

saifu o wasurete shimaimashita

I'm afraid I left my wallet behind.

Key takeaways

te-form + しまう seals an action as finished and irreversible.
One core, two feelings: wanted outcome → completion ("done, all the way"); unwanted outcome → regret ("ended up …-ing, oops"). Context and intonation choose.
Contractions: て → ちゃう, で → じゃう (past ちゃった / じゃった) — mechanically tied to the te-form's final kana. Always be able to expand them back to てしまう.
ちゃう / じゃう are informal; keep the full てしまいました for polite or written contexts.
Don't translate away the nuance: rendering 忘れてしまった as a flat "I forgot" throws out exactly what しまう was there to add.

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