Translation guide
Expressing that an action was done accidentally, unintentionally, or by mistake. Japanese uses verb endings, adverbs, and set phrases rather than a single word like 'accidentally'.
The most common way to say you did something accidentally, without intending to. Attach to the verb stem.
Attach to the te-form of a verb. Often contracted to 〜ちゃう (casual) or 〜でしまう/〜じゃう. Expresses that an action happened unintentionally or regrettably.
ケーキを全部食べてしまった。
I accidentally ate all the cake.
財布を落としちゃった。
I accidentally dropped my wallet.
Casual contraction of 〜てしまう. Used in everyday speech.
また遅刻しちゃった。
I accidentally ended up late again.
Used after verbs with te-form ending in で (e.g., 読んで → 読んでしまう).
その本を読んでしまった。
I accidentally read that book (maybe I shouldn't have).
Emphasizing that the action was a mistake or done carelessly.
Adverb meaning 'carelessly', 'absentmindedly', 'without thinking'. Often used with 〜てしまう.
うっかり約束を忘れてしまった。
I accidentally forgot the appointment.
うっかり電車を乗り過ごした。
I accidentally missed my stop.
Te-form of 間違う (to make a mistake). Means 'by mistake' or 'erroneously'.
間違ってメールを消してしまった。
I accidentally deleted the email.
Adverb meaning 'unintentionally', 'in spite of oneself'. Often used for actions done out of habit or impulse.
つい食べ過ぎてしまった。
I accidentally ate too much (couldn't help it).
The action happened without the person realizing it at the time.
Means 'unconsciously', 'without knowing'. Suggests the action happened gradually or without awareness.
知らず知らずのうちに、悪い習慣が身についてしまった。
I accidentally picked up a bad habit without realizing it.
Used in formal writing or speeches to express accidental actions.
Formal/literary adverb meaning 'in error', 'mistakenly'.
過って重要な書類を破棄してしまった。
I accidentally destroyed an important document.
Same reading, often written with kanji 誤. Means 'erroneously', 'by mistake'.
誤ってデータを削除しました。
I accidentally deleted the data.
There is no single Japanese word that directly corresponds to 'accidentally' in all contexts. The most natural way is to use the verb ending 〜てしまう (or its contractions) to convey that an action was unintentional or regrettable.
〜てしまう is a verb ending that adds the nuance of 'unintentionally' or 'completely'. うっかり is an adverb meaning 'carelessly' and is often used together with 〜てしまう to emphasize the accidental nature. You can use 〜てしまう alone for most situations.
うっかり忘れてしまった。
I accidentally forgot (carelessly).
忘れてしまった。
I forgot (accidentally/regrettably).
間違って違う人にメールを送ってしまった。
I accidentally sent the email to the wrong person.
彼女はうっかりシャツにコーヒーをこぼしてしまった。
She accidentally spilled coffee on her shirt.
Means 'before one knows it', 'unnoticed'. Often used for changes or actions that happen over time.
いつの間にか寝てしまった。
I accidentally fell asleep without noticing.