Look at these two sentences:
A. 本を読んでしまった。 → I finished reading the book.
B. 財布を忘れてしまった。 → I accidentally forgot my wallet.
Same てしまう, but A means "completely finished" and B means "accidentally messed up." How do you tell which meaning it is? Context and the speaker's feelings.
Usage 1: Complete Thoroughly
The first meaning of てしまう is "the action is completely, thoroughly finished — no going back."
-
本を全部読んでしまった。 → I finished reading the entire book.
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宿題をもうやってしまった。 → I've already finished the homework.
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ケーキを一人で食べてしまった。 → I ate the whole cake by myself.
Nuance: The action was done completely, with nothing left.
Usage 2: Accidentally / Regrettably
The second meaning is "I accidentally did something I didn't mean to" or "something regrettable happened."
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友達の辞書をなくしてしまった。 → I accidentally lost my friend's dictionary.
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10時まで寝てしまった。 → I ended up sleeping until 10. (Didn't mean to sleep that long.)
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猫が死んでしまった。 → The cat died. (Feeling sad and regretful.)
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約束を忘れてしまった。 → I accidentally forgot our appointment.
Nuance: It wasn't intentional, or the speaker feels regret or disappointment.
How to Tell Which Meaning
| Clue | Completion | Regret |
|---|---|---|
| Positive action | ✅ Finished reading, writing | |
| Negative action | ✅ Lost, forgot | |
| Context suggests "accidentally" | ✅ | |
| Emphasis on "completely, all of it" | ✅ |
Quick rule: Good thing done = completion. Bad thing happened = regret.
Casual Contraction: ちゃう / じゃう
In spoken Japanese, てしまう is almost never said in full — it contracts to ちゃう (て → ちゃ) or じゃう (で → じゃ):
| Formal | Casual |
|---|---|
| 食べてしまう | 食べちゃう |
| 食べてしまった | 食べちゃった |
| 読んでしまう | 読んじゃう |
| 読んでしまった | 読んじゃった |
| 忘れてしまった | 忘れちゃった |
In daily conversation: 「あ、忘れちゃった！」 is much more natural than 「忘れてしまった」.
Conjugation
てしまう attaches to the て-form of verbs:
| Verb | て-form | + しまう |
|---|---|---|
| 食べる | 食べて | 食べてしまう |
| 読む | 読んで | 読んでしまう |
| 忘れる | 忘れて | 忘れてしまう |
| 行く | 行って | 行ってしまう |
Self-Test
Q1. Is 「宿題を全部やってしまった」 completion or regret?
Show answer
Completion. Finishing all the homework is a positive thing. It emphasizes "completely done."
Q2. Is 「電車で寝てしまった」 completion or regret?
Show answer
Regret. Accidentally fell asleep on the train — didn't mean to.
Q3. What is the formal form of 「食べちゃった」?
Show answer
食べてしまった. ちゃった is the casual contraction of てしまった.
Summary
- てしまう has two meanings: thorough completion and accidental/regrettable
- Good thing done = completion; bad thing happened = regret
- Casual contraction: てしまう → ちゃう, でしまう → じゃう
- Conjugation: verb て-form + しまう