Sorry if this was asked before, but I was wondering about these three words that supposedly all mean "why". Are they absolutely interchangeable or are there some rules for their usage? In some of the anime I have watched I noticed that なぜ was used by either guys or tomboys, with girly girls preferring the other two. Is there a difference in formality/politeness or was that just a coincidence (or poor observation on my behalf)? And do they all have the exact same meaning? I am asking this because in my native Russian there are two words that get translated as "why", "почему" and "зачем", but the latter is closer in meaning to "what for", i.e. only applicable to purposeful actions.

なんで is the most casual of the three; you only find it in speech.

なぜ is odd because it's the normal "why" used in formal language, but then in speech it has a slightly rougher sound than どうして, although there's really not that much of a difference.

My first Japanese friend would always correct my なぜs into なんでs and I'm not really sure what the specific rule is but it kind of got drilled into my head. I never really use なぜ at all now.

In Japan I heard なぜ maybe a few times. I heard 何で all the time.

Never had a なんで corrected to なぜ either.

Think of it this way

なぜ = Wherefore? Thereby?

なんで = How come?

どうして = Why?

I would only use the first one writing an essay. The other two, maybe fifty fifty.
In other words, don't say なぜ and you can't go wrong.

Mennon Wrote:Think of it this way

なぜ = Wherefore? Thereby?

なんで = How come?

どうして = Why?

I would only use the first one writing an essay. The other two, maybe fifty fifty.
In other words, don't say なぜ and you can't go wrong.

なぜ is not common in casual speech but is certainly used in more polite/formal speech. In these situations I would think なんで and どうして would sound quite inappropriate and なぜ would, by far, be the best choice.

Thanks for the responses, everyone! They made it much clearer.