Translation guide
Ways to express lack of knowledge, unawareness, or ignorance in Japanese, organized by what kind of 'not knowing' you mean.
I don't know. (polite)
Polite and safe for most situations
You don't know a fact, answer, or piece of information.
The most direct and common way to say you don't know something. Use with a noun or noun phrase.
I don't know that news.
I don't know his phone number.
General 'I don't know' or 'I don't understand'. More casual and versatile than 知らない. Can also imply you haven't figured something out.
I don't know the answer.
Polite form of 知らない. Use in formal situations or with strangers.
Sorry, I don't know.
Polite form of わからない. Safe for most polite contexts.
I'm sorry, but I don't know.
You were not aware something happened or existed.
Expresses that you didn't notice or realize something. Often used for past unawareness.
You lack knowledge in a certain field or are generally uninformed.
Means 'to be ignorant'. Can sound harsh if used about others. Often used in self-deprecation.
You act as if you don't know something, often deliberately.
To pretend not to know. Common in everyday speech.
知らない means you lack information or have never encountered something. わからない means you don't understand or can't figure something out. In many casual situations they overlap, but わからない is softer and more common when you're unsure.
I don't know his address. (I never learned it.)
何が起こったのかわからない。
I don't know what happened.
I didn't realize he had come.
Past tense of 知らない, meaning you didn't know about something before (but maybe you do now).
I wasn't aware of that problem.
Casual 'I didn't know'. Often used when you just learned something surprising.
Oh, really? I didn't know!
I am ignorant about that field.
Literally 'have no knowledge'. Neutral way to say you lack expertise.
I don't have legal knowledge, so I'll consult an expert.
Emphatic 'I don't know anything'. Can be used when you feel completely clueless.
I don't know anything.
He pretended not to know.
To play dumb, feign ignorance. Often used when someone is being evasive.
Don't play dumb, tell me the truth.
To feign innocence, especially when guilty. Slightly old-fashioned or dramatic.
He feigned innocence and said he knew nothing.
I don't understand this problem. (I can't solve it.)
English 'not knowing' as a gerund doesn't directly map to a single Japanese noun. Use verb phrases like 知らないこと or 無知 depending on context. Don't try to nominalize 知らない as a standalone noun without こと or の.