Translation guide
In Japanese, expressing 'accidentally' depends on whether the action was unintentional, a mistake, or something that happened by chance. The most common way is using the verb ending 〜てしまう, which conveys that something happened unintentionally or regrettably. Other expressions like うっかり, 偶然, and 間違って are used for specific nuances.
Express that an action was done without intention, often with a sense of regret or unexpectedness.
I accidentally ate all the cake.
I accidentally forgot my wallet.
An adverb meaning 'carelessly' or 'absentmindedly'. Used when the accident is due to inattention or forgetfulness.
I accidentally missed my stop.
I accidentally forgot his name.
An adverb meaning 'without thinking' or 'in spite of oneself'. Often used for actions done out of habit or impulse.
I accidentally ate too much.
I accidentally told the secret.
Express that an action was done incorrectly or by error.
Express that something happened by chance, without planning.
An adverb or na-adjective meaning 'by chance' or 'coincidentally'. Used for unexpected but not necessarily negative events.
I met him accidentally (by chance).
A casual adverb meaning 'by chance' or 'coincidentally'. Similar to 偶然 but more conversational.
Express that an action was made to look accidental but was actually intentional.
An adverb meaning 'in a contrived manner' or 'as if on purpose'. Used when someone pretends something was an accident.
He knocked over the cup accidentally on purpose.
〜てしまう is a grammatical construction that attaches to verbs and can express unintentional actions in general. うっかり is an adverb specifically for careless mistakes due to inattention. You can combine them: うっかり忘れてしまった (I carelessly forgot).
I accidentally forgot (carelessly).
偶然 implies a chance occurrence, not a mistake. If you delete a file by mistake, use 間違って or 〜てしまう, not 偶然.
I accidentally sent personal information.
I accidentally found that book.
たまたま通りかかった。
I happened to pass by.