Grammar · N4
〜てしまう: completion and unintended result
Mark an action as fully completed or express regret/unintended outcome.
In short
Verb て-form + しまう can emphasize that something is completely done. In contexts involving mistakes or unwanted events, it often adds a feeling like “ended up…” or “unfortunately…”.
How it works
財布を忘れてしまいました naturally suggests an unfortunate realization. 全部読んでしまいました can simply emphasize finishing the whole thing.
Useful contrast
Casual speech commonly compresses てしまう→ちゃう and でしまう→じゃう.
Examples
電車で寝てしまいました。
I accidentally fell asleep on the train.
本を全部読んでしまった。
I finished the whole book.
Common pitfall
Do not assume しまう always expresses regret. Completion may be neutral or even positive.