〜ちゃう / 〜じゃう — informal 〜てしまう / 〜でしまう
〜ちゃう is the spoken contraction of 〜てしまう, and 〜じゃう the contraction of 〜でしまう. Both add a layer of meaning to a verb: completion ('finish doing it'), regret ('oops, did it'), or unintentional action ('ended up doing it'). The full forms are perfectly grammatical in writing and polite speech, but in everyday conversation natives almost always squeeze them down — 食べて しまった becomes 食べちゃった, 読んで しまった becomes 読んじゃった. Master the contraction rule and you've unlocked a feature that appears in every casual sentence.
How the contraction works
Take the 〜て / 〜で form of any verb and replace the ending. 〜て しまう → 〜ちゃう. 〜で しまう → 〜じゃう. The rule mirrors the て-form voicing: verbs whose te-form is 〜て use ちゃう; verbs whose te-form is 〜で use じゃう. Examples: 食べて しまう → 食べちゃう ('eat it all up'), 書いて しまう → 書いちゃう ('finish writing'), 読んで しまう → 読んじゃう ('finish reading'), 死んで しまう → 死んじゃう ('die / pass away'). The rest of the verb conjugates as a regular u-verb: 食べちゃう → 食べちゃった (past), 食べちゃわない (negative), 食べちゃおう (volitional). Past tense is by far the most common form — 〜ちゃった / 〜じゃった.
Meaning 1: completion
The first meaning is completion — finishing an action through to the end, often with a sense of polishing it off. ケーキ を ぜんぶ 食べちゃった。 = 'I ate all the cake.' 本 を 一日 で 読んじゃった。 = 'I finished reading the book in a day.' しゅくだい は もう やっちゃった よ。 = 'I already did the homework.' English 'I finished V-ing' or 'I went ahead and V-ed' captures the flavor. The completion sense often shades into pride or relief: getting something done. Compare neutral 食べた (just 'I ate') with 食べちゃった ('I ate it up / I went ahead and ate'). Same fact, different framing.
Meaning 2: regret / unintentional
The second meaning is regret or unintended outcome. しゅくだい を 忘れちゃった! = 'I forgot my homework!' コップ を 落として、われちゃった。 = 'I dropped the cup and it broke.' 電車 で ねちゃった。 = 'I fell asleep on the train.' Often there's an 'oops' feeling — you didn't mean to, or you regret it. Tone of voice and context decide whether 〜ちゃった is proud completion ('ate it all up!') or rueful slip ('ate it all up…'). The same form does both. Compare: ぜんぶ 飲んじゃった! (cheerful: drank it all!) vs. ぜんぶ 飲んじゃった… (rueful: I drank it all, oops). Casual Japanese leans heavily on this dual-flavor ambiguity.
Conjugation examples — group 1, 2, irregular
Group-1 (u-verbs) — drop the 〜て/〜で of the te-form and tack on 〜ちゃう / 〜じゃう. 書く → 書いて → 書いちゃう, 話す → 話して → 話しちゃう, 読む → 読んで → 読んじゃう, 遊ぶ → 遊んで → 遊んじゃう, 死ぬ → 死んで → 死んじゃう. Group-2 (ru-verbs) — same rule, always ちゃう: 食べる → 食べて → 食べちゃう, 見る → 見て → 見ちゃう, わすれる → わすれて → わすれちゃう. Irregular: する → して → しちゃう, 来る → 来て → 来ちゃう. The full-form alternates 〜てしまう / 〜でしまう are always swappable — 食べちゃった = 食べて しまった, 読んじゃった = 読んで しまった.
Register warning — keep it casual
〜ちゃう / 〜じゃう is spoken-only. Do not use it in: business email, news writing, formal speeches, polite conversation with someone above you in social rank. In those contexts, use the full 〜てしまう / 〜でしまう (or just plain past tense if completion / regret aren't critical). しゅくだい を 忘れて しまいました。 = 'I forgot my homework' (polite). しゅくだい を 忘れちゃった。 = same meaning, casual. Among friends, family, peers, social media — 〜ちゃう is everywhere. There's also a tougher / more masculine variant 〜ちまう / 〜じまう (e.g., 食べちまった) that you'll hear in older speech, anime tough guys, and Tokyo working-class register; recognize it but don't reach for it as a beginner.
