The ～てしまう Form in Japanese: Completion, Regret, and the Casual ちゃう / じゃう

The ～てしまう form attaches to a verb's te-form. It carries two readings that can look unrelated in English: completion ("all the way") and regret ("ended up"). Both spring from a single Japanese idea.¹ It is a JLPT N4 grammar point. In casual speech, it contracts to ～ちゃう / ～じゃう, the forms learners usually hear most.²³

Overview

～てしまう is one of the te-form auxiliaries: here, しまう attaches to another verb's te-form to add meaning.¹² Grammar calls this kind of attached verb a 補助動詞 (hojo-dōshi, "auxiliary verb").

It belongs to the same family as ～ている, ～てある, and ～ておく. On the aspect map, it is the perfective or completive entry.² The dictionary records completion and regret as two sub-senses of one auxiliary entry, not two separate grammar points.¹

One form, two readings, one source idea

The completion reading ("finished it off") and the regret reading ("ended up doing it") are not coincidental homonyms. They are two faces of the same auxiliary, both inherited from the lexical verb しまう.¹ The rest of this article traces how.

Where it sits

～てしまう sits alongside the other te-form auxiliaries a learner meets around N4.² Each one takes a verb's te-form and adds an aspectual or modal layer. ～てしまう adds completion or regret.

In kana-vs-kanji terms, the auxiliary use is conventionally written in kana (てしまう), even though the underlying lexical verb has kanji.¹

しまう as a lexical verb first

Before it became an auxiliary, しまう was an ordinary verb. デジタル大辞泉 records three core lexical senses: to bring an ongoing matter to an end ("finish work for the day"), to stop or close up a business, and to put used or valued things away in their proper place.¹

The dictionary also lists a settling-accounts sense, especially closing the year-end books.¹

The thread running through all of these is "bring something to a definitive close": finish the activity, shut the shop, put the object away for good, close the books.¹ This is the seed meaning the auxiliary inherits. The same root surfaces in the noun しまい / おしまい ("the end").¹

もう店みせを仕舞しまう時間じかんだ。¹
"It's already time to close up the shop."

おもちゃを箱はこに仕舞しまう。¹
"I put the toys away in the box."

These two examples use しまう as a full lexical verb, written in kanji 仕舞う, not as the auxiliary.¹ They establish the "close / put away / finish off" meaning that ～てしまう later carries in abstract form.

Form and conjugation

Building the base: te-form + しまう

The formation is simple: take a verb's te-form and add しまう.²³⁴ The verb decides whether the te-form ends in て or で, not しまう.

Verbs whose plain te-form is voiced (ends in で) keep that で and add しまう: 飲む → 飲んで → 飲んでしまう, 読む → 読んで → 読んでしまう.³⁵ The で in でしまう is inherited from the te-form, not a separate rule. JLPTsensei states the formation as "Verb (て form) + しまう" with the voiced variant "Verb (で form) + しまう," where で applies to verbs ending in ぶ, む, ぬ, ぐ.³

ケーキを全部ぜんぶ食たべてしまった。⁶
"I ate the whole cake."

ジュースを全部ぜんぶ飲のんでしまった。³
"I drank up all the juice."

In the first example, 食べる takes a voiceless te-form (食べて), so the base is てしまう.⁶ In the second, 飲む takes a voiced te-form (飲んで), so the base is でしまう. The voicing is carried from the te-form.³

Conjugating しまう itself

しまう is a regular 五段 (godan / u-verb).²³ Only the auxiliary changes form. The te-form in front of it never changes.

The past ～てしまった and polite past ～てしまいました are the most useful forms, because the action reported is usually already done.⁴⁶

Form	Build	Reading	Function
～てしまう	te-form + しまう	-te shimau	plain non-past2
～てしまいます	te-form + しまいます	-te shimaimasu	polite non-past2
～てしまった	te-form + しまった	-te shimatta	plain past (most common)4
～てしまいました	te-form + しまいました	-te shimaimashita	polite past4
～てしまわない	te-form + しまわない	-te shimawanai	plain negative2
～てしまおう	te-form + しまおう	-te shimaō	volitional ("let's just get it done")2

宿題しゅくだいを全部ぜんぶやってしまいました。⁴
"I finished all my homework (got it all done)."

今日中きょうじゅうに書かいてしまおう。²
"Let me just get it written by the end of today."

The two readings, from one source

Completion: doing it all the way / for good

デジタル大辞泉 records the completion sense as expressing that the action is completed and the state is thoroughly reached.¹ Pedagogy labels it 完了 (kanryō, "completion"): the action is carried out fully, often "all of it" or "to the end."⁴⁶

The tone here can be neutral or even satisfied. It is not necessarily negative.³⁶ Adverbs that lean the reading toward completion include 全部 ("all"), もう ("already"), and すっかり ("completely").⁴

借かりた漫画まんがを全部ぜんぶ読よんでしまった。⁴
"I read all the way through the manga I'd borrowed."

この映画えいがのシリーズは一日いちにちで全部ぜんぶ見みてしまいました。⁴
"I watched the entire movie series in a single day."

Regret and the unintended: ended up / accidentally

デジタル大辞泉 records the regret sense as expressing that a situation comes about even though one did not intend it.¹ Pedagogy labels it 残念 / 後悔 (zannen / kōkai, "regret / disappointment"): the speaker did something they should not have, did not mean to do, or wishes had not happened. The result cannot be undone.⁴⁶

絵でわかる日本語 paraphrases the nuance as "did it inadvertently, even though one shouldn't have."⁶ Adverbs that lean the reading toward regret include うっかり ("carelessly"), つい ("without meaning to"), and 無意識に ("unconsciously"). A nearby apology such as ごめん or すみません often accompanies it.³

傘かさを忘わすれてしまった。³
"I went and forgot my umbrella."

料理りょうりをしている時ときに、指ゆびを切きってしまった。⁴
"I accidentally cut my finger while I was cooking."

どこかで財布さいふを落おとしてしまった。⁴
"I went and lost my wallet somewhere."

How to tell which reading is meant

The same form carries both readings. You judge the intended one from the verb's meaning, the surrounding context, and any adverbs.³⁶ A quick heuristic comes straight from the sources: 全部 / もう / すっかり tilt toward completion, while うっかり / つい / 無意識に (and a nearby apology) tilt toward regret.⁴⁶

The common thread is irreversibility: the action is now done and cannot be taken back.¹⁷ Completion foregrounds "done all the way." Regret foregrounds "done, and I wish it weren't." This is why one form covers both.

The following diagram shows how the same base form resolves to one reading or the other based on the cues around it.⁴⁶

Grammaticalization research also treats the two readings as one form. Isshiki (2011) argues that the perfective / completion reading is "no more than a secondary interpretation" arising in context. Subjective senses such as regret and "sweeping it all away," along with intersubjective senses such as excuse-making and covering embarrassment, emerge pragmatically from the same auxiliary as it grammaticalized from the lexical verb.⁷

ケーキを食たべてしまった。⁶
"I ate it all up." (completion) / "I went and ate the cake, I shouldn't have." (regret)

Out of context, this sentence is genuinely ambiguous. Adverbs or the situation fix the reading.⁶ Add 全部 and it reads as completion. Add ダイエット中なのに ("even though I'm dieting") and it reads as regret.

The casual contractions ちゃう and じゃう

The rule: てしまう → ちゃう, でしまう → じゃう

In casual speech, ～てしまう contracts to ～ちゃう, and the voiced ～でしまう contracts to ～じゃう.²³⁵ The te-form's て or で decides ち or じ, just as it decides the base form.³

The mapping is direct: 食べてしまう → 食べちゃう, and 飲んでしまう → 飲んじゃう.²³ Any verb whose te-form is voiced (でしまう) contracts to じゃう. The rest contract to ちゃう.³

ごめん！ジュースを全部ぜんぶ飲のんじゃった！³
"Sorry! I drank all the juice!"

疲つかれが出でたので、寝ねちゃった。³
"I was worn out, so I ended up falling asleep."

Past, polite, and the rest of the paradigm

ちゃう and じゃう conjugate as regular 五段 verbs in their own right.³⁵ The past ～ちゃった / ～じゃった is the form you hear most often.³

Full form	Contracted	Function
～てしまう	～ちゃう	plain non-past3
～てしまった	～ちゃった	plain past (most heard)3
～てしまいます	～ちゃいます	polite non-past3
～てしまおう	～ちゃおう	volitional2
～でしまう (voiced)	～じゃう	voiced-stem non-past35
～でしまった (voiced)	～じゃった	voiced-stem past35

レポートを忘わすれちゃいました。⁶
"I went and forgot the report."

つい笑わらっちゃった。⁶
"I couldn't help laughing."

Register: where ちゃう belongs

～ちゃう / ～じゃう are casual-speech and informal-writing forms. The sources mark them explicitly as 口語 ("colloquial / spoken").⁶⁵ In neutral or polite contexts, use ～てしまいます. In formal writing, spell out しまう in full.²⁶

Keep ちゃう out of formal writing

The contraction sits on the same casual register tier as English "wanna" or "gonna." It is correct casual Japanese, but out of place in formal writing, business documents, or exam composition.⁶⁵ Use ～てしまう or ～てしまいました there instead.

Nuance and usage contexts

Softening and self-deprecation

Beyond literal regret, ～てしまう (and casual ～ちゃう) has a social function: it frames an action as something the speaker could not help, softening a confession or admission.⁷⁶ Isshiki (2011) groups these uses under the form's "de-intentionalizing" function. In other words, it presents the act as not fully intended and can cover excuse-making, embarrassment, and hedging.⁷

This is why つい笑っちゃった ("I just couldn't help laughing") sounds like light self-deprecation rather than literal remorse.⁷⁶

ダイエット中ちゅうなのに、ケーキを食たべちゃった。⁶
"Even though I'm on a diet, I went and ate the cake."

Irreversibility as the common thread

Both readings agree that the action is sealed and cannot be undone.¹⁷ This shared "done, no going back" idea connects the lexical しまう ("put it away, close it up, finish it off") to both auxiliary readings: completion and regret.

That is why "ate it all up" (completion) and "broke it by accident" (regret) take the same form. In each case, the event is now a closed, finished fact.¹

コップが割われてしまった。³
"The cup got broken, and that's that."

Good to know

しまった! as a standalone "oh no"

The interjection しまった! that learners hear constantly is the past tense of しまう, frozen into a standalone word. デジタル大辞泉 defines it as a word one blurts out after failing or making a mistake. It gives the derivation as the 連用形 of the verb しまう plus the perfective auxiliary た.⁸

Hearing しまった！ and ～てしまった as the same word makes the regret nuance easier to feel. The "oh no" interjection is literally the regret reading with the main verb dropped.⁸

じゃう (verb auxiliary) versus じゃ (from では)

The じゃ in じゃない / じゃなかった is a contraction of では. It is different from the verb auxiliary じゃう. A learner who parses 飲んじゃう as 飲ん + じゃ(=では) + う has the wrong analysis.

The correct parse is 飲んじゃう = 飲んでしまう: te-form 飲んで plus auxiliary しまう, contracted.³ Position helps keep the two apart. じゃう only ever attaches to a voiced te-form (んじゃう, いじゃう), whereas the では-derived じゃ appears before ない / なかった / だめ. Same two kana, unrelated grammar.

ジュースを飲のんじゃう。³
"I'll (end up) drinking the juice."

The で in 飲んでしまう is inherited, not new

A verb's te-form entirely decides whether it takes てしまう or でしまう, and therefore ちゃう or じゃう. The voiced で comes from the 五段 sound-change groups: ぶ・む・ぬ → んで (遊ぶ → 遊んで, 飲む → 飲んで, 死ぬ → 死んで) and ぐ → いで (泳ぐ → 泳いで). Everything else takes て.³

A learner who already knows 飲んで needs no new でしまう table. しまう just attaches, and the contraction voices in parallel: 飲んでしまう → 飲んじゃう.³ The full て-vs-で formation is the standard te-form sound change covered in the te-form construction rules.

Shut the drawer: a mnemonic for both readings

しまう literally means "put it away, close it up, finish it off."¹ Picture sliding a drawer shut: once it is closed, the matter is done. Sometimes that is neutral completion ("got it all put away"). Sometimes it is regret ("oops, and now it is shut for good").¹⁷

The image ties both readings to the same lexical verb and to the shared idea of irreversibility. That helps them stop feeling like two unrelated grammar points.¹⁷

Using ～てしまう when no completion or regret is intended

～てしまう is not a neutral past tense. It always adds either "all the way / for good" (completion) or "ended up / regrettably" (regret).¹⁴⁶ A learner who says 宿題をしてしまった intending a flat "I did my homework" accidentally adds an "and that's finally over with" or "oops" coloring.

For a neutral report, use the plain past or a verb like 終える ("finish") instead.

宿題しゅくだいを終おえました。⁶
"I finished my homework."

See also

• The ～てくる / ～ていく Form in Japanese: Directional and Temporal Movement
• The Te-Form Benefactive: 〜てあげる, 〜てくれる, 〜てもらう
• ～出す, ～切る, ～込む, ～直す in Japanese: V2 Aspect Suffixes (Sudden Onset, Completion, Inward/Depth, Redo)
• The Plain Volitional Form ～よう / ～おう: How to Say "Let's" and "I Will" in Japanese
• ～てもいい / ～てはいけない: How to Ask Permission and State Prohibition in Japanese
• ～始める, ～終わる, ～続ける in Japanese: Beginning, Ending, and Continuing an Action

References

Footnotes

1. デジタル大辞泉 (Shogakukan). Entry「しまう【仕舞う／▽終う／▽了う】」, lexical and 補助動詞 senses, via Weblio. https://www.weblio.jp/content/%E3%81%97%E3%81%BE%E3%81%86 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰ ↩²¹

2. Bunpro. Grammar point「てしまう・ちゃう」(JLPT N4). https://bunpro.jp/grammar_points/%E3%81%A6%E3%81%97%E3%81%BE%E3%81%86-%E3%81%A1%E3%82%83%E3%81%86 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵

3. JLPTsensei. "JLPT N4 Grammar: てしまう / ちゃう (te shimau / chau) Meaning." https://jlptsensei.com/learn-japanese-grammar/%E3%81%A6%E3%81%97%E3%81%BE%E3%81%86-te-shimau-%E3%81%A1%E3%82%83%E3%81%86-meaning/ ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰ ↩²¹ ↩²² ↩²³ ↩²⁴ ↩²⁵ ↩²⁶ ↩²⁷ ↩²⁸ ↩²⁹

4. 日本語NET (nihongokyoshi-net.com).「【JLPT N4】文法・例文：〜てしまう」. https://nihongokyoshi-net.com/2020/01/02/jlptn4-grammar-teshimau/ ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵

5. 毎日のんびり日本語教師 (mainichi-nonbiri.com).「【N4文法】～てしまう／ちゃう／でしまう／じゃう」. https://mainichi-nonbiri.com/grammar/n4-teshimau/ ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

6. 絵でわかる日本語 (edewakaru.com).「〜てしまう（残念・後悔）｜日本語能力試験 JLPT N4」. https://www.edewakaru.com/archives/16372216.html ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰ ↩²¹ ↩²²

7. 一色舞子 (Isshiki, Maiko).「日本語の補助動詞「‐てしまう」の文法化：主観化、間主観化を中心に」.『日本研究』(Japanese Studies) vol. 15, Hokkaido University, 2011. https://eprints.lib.hokudai.ac.jp/dspace/handle/2115/45277 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

8. デジタル大辞泉 (Shogakukan). Entry「しまった」(感動詞), with derivation note, via Weblio. https://www.weblio.jp/content/%E3%81%97%E3%81%BE%E3%81%A3%E3%81%9F ↩ ↩²