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.
I accidentally ate all the cake.
I accidentally dropped my wallet.
Casual contraction of 〜てしまう. Used in everyday speech.
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).
I accidentally read that book (maybe I shouldn't have).
Casual contraction of 〜でしまう.
I accidentally drank it.
Emphasizing that the action was a mistake or done carelessly.
The action happened without the person realizing it at the time.
Used in formal writing or speeches to express accidental actions.
Formal/literary adverb meaning 'in error', 'mistakenly'.
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.
I accidentally forgot (carelessly).
I accidentally sent the email to the wrong person.
Adverb meaning 'carelessly', 'absentmindedly', 'without thinking'. Often used with 〜てしまう.
I accidentally forgot the appointment.
I accidentally missed my stop.
Te-form of 間違う (to make a mistake). Means 'by mistake' or 'erroneously'.
I accidentally deleted the email.
Adverb meaning 'unintentionally', 'in spite of oneself'. Often used for actions done out of habit or impulse.
I accidentally ate too much (couldn't help it).
Means 'unconsciously', 'without knowing'. Suggests the action happened gradually or without awareness.
I accidentally picked up a bad habit without realizing it.
Means 'before one knows it', 'unnoticed'. Often used for changes or actions that happen over time.
I accidentally fell asleep without noticing.
I accidentally destroyed an important document.
Same reading, often written with kanji 誤. Means 'erroneously', 'by mistake'.
I accidentally deleted the data.
忘れてしまった。
I forgot (accidentally/regrettably).
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.