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N5Lesson notes · Oct 15, 2026

Why in Japanese: なんで vs どうして vs なぜ

Japanese has three common ways to ask "why": なんで (nande) is casual and everyday, どうして (doushite) is neutral and works in speech and writing, and なぜ (naze) sounds more formal and analytical. For asking a reason all three translate as "why," so the choice is register; whether a question sounds curious or accusing depends on context and tone, not on the word alone.

This is the "why" companion to what, where and when and how in Japanese.

The basics

Which word for "why" should I use?

Use どうして when you are unsure: it is neutral, it fits speech and writing, and it is not tied to one relationship. なんで is what friends and family say all day. なぜ reads as more formal or analytical, so it is common in essays, news and prepared talks.

なんで来こなかったの? — Why didn't you come? (casual, friends and family) どうして来こなかったんですか。 — Why didn't you come? (polite, neutral; between people who are not close it can still sound like asking for an account of the absence) なぜ日本人にほんじんは家いえに入はいるとき靴くつを脱ぬぐのですか。 — Why do Japanese people take their shoes off when entering a house? (formal, the style of an article or survey)

Readings: 来らい(こ)なかった, 日本人にほんじん(にほんじん), 家いえ(いえ), 入にゅう(はい)る, 靴くつ(くつ), 脱だつ(ぬ)ぐ.

How do I make each one polite?

It depends on what follows the "why." Alone, add です: どうして? → どうしてですか。 With a verb, the usual polite form is the んです form: どうして行いくんですか. With a noun or na-adjective, the explanation form needs な: どうして休やすみなんですか. Never attach です to a plain verb (どうして行いくですか is wrong).

The plain の and んだ forms are not polite by themselves, and です does not simply sit on top of both. The conversions are 行いくの? → 行いくんですか and 行いくんだ → 行いくんです (行いくのですか is also correct; んですか is what you hear more in conversation). なんでですか is common in polite speech, but it still carries the casual flavor of なんで.

What does the の / んです ending add?

It presents the question as asking for an explanation or background behind a situation: "there is a reason behind this, what is it?" どうして遅おくれましたか also asks for a reason, so んです is not what makes a question a real question. It adds a link to the situation, and it can be gentle or pushy depending on how you say it. Casual questions do not need の either: なんでそう思おもう? and なんで知しってるの? are both ordinary.

どうして遅おくれたんですか。 — Why were you late? (polite, asks for the background; with a sharp tone it sounds like pushing for an excuse) なんでそう思おもう? — Why do you think so? (casual, no の, completely normal)

Beyond the basics

Why do my "why" questions sound like blame?

Because asking someone to justify their own action can feel like asking for a defense, and no single word or ending prevents that. Politeness and blame are separate axes: a polite sentence can interrogate, and a casual one can be warm. What changes the effect is the relationship, your voice, the situation, and whether the other person knows why you are asking.

Compare the same sentence frame with only the question word changed:

なんで予定よていを変かえたんですか。 — Why did you change the plan? (casual-leaning, can sound sharp) どうして予定よていを変かえたんですか。 — Why did you change the plan? (neutral, the usual choice) なぜ予定よていを変かえたんですか。 — Why did you change the plan? (analytical; sounds like a formal inquiry)

The most reliable fix is to say what the answer is for:

今後こんごの調整ちょうせいのために、予定よていを変更へんこうした理由りゆうを教おしえてもらえますか。 — For future scheduling, could you tell me why the plan was changed? (polite, states your purpose; still a request for an explanation, so it can carry pressure) なんでそんなことするの? — Why would you do something like that? (casual; with a flat or rising-angry voice this is criticism, with a laugh it can be teasing)

A short opener such as ちょっと聞きいてもいいですか tells the person a question is coming, but it does not remove the pressure by itself. Try the same line aloud with worried, curious and annoyed voices; the words stay the same and the meaning does not.

Is なぜ used in speech?

Yes. なぜ is common in spoken Japanese when the speaker sounds analytical, thoughtful or formal: in a meeting, a lecture, an interview, or when musing aloud.

なぜそう思おもうんですか。 — Why do you think that? (polite, analytical, fine in conversation) なぜでしょうね。 — I wonder why. (polite, thoughtful, talking to yourself or the room)

Can なんで mean something other than "why"?

Yes, two things. 何なんで also asks about means: 何なんで行いくの? can be "Why are you going?" or "By what means are you going?", and context decides, not the ending. After a noun, casual 〜なんで can be a short form of なので.

何なんで行いくの? — Why are you going? / How are you getting there? (casual; the situation tells you which) 雨あめなんで、中止ちゅうしです。 — Since it's raining, it's canceled. (casual-to-colloquial, a contraction of なので)

How does "why" sound in anime and casual speech?

Performance stretches the three words. なんで!? is the loud, shocked version with a sharp stop. なんでだよ is exasperated or hurt, typically in rough, informal speech. どうして… said quietly and trailing off is a sad, often rhetorical "why." なぜだ…! is the dramatic one: it sounds like narration of one's own confusion, so depending on delivery it can come across as serious, theatrical or villain-like; that is an impression of the performance, not a rule.

In Kansai speech, なんでやねん is a stock comic pushback: the tsukkomi reply to a joke or a silly statement, not a neutral question.

How to answer a "why" question

How do I say "because" in conversation?

Often you just state the reason with 〜んです, 〜からです or 〜ので. You do not need a word for "because" at the start.

電車でんしゃが止とまったんです。 — The train stopped. (polite, a common short answer to どうして遅おくれたんですか) 電車でんしゃが止とまったからです。 — It's because the train stopped. (polite, a clean, slightly firmer answer) 電車でんしゃが止とまったので… — The train stopped, so... (a trailing-off that is fine in polite conversation; the result is left unsaid, which softens the reason. For a fully polite clause use 止とまりましたので…)

For the choice between the two reason particles, see から vs ので.

Is なぜなら…からです wrong in conversation?

Not wrong, only formal. This is a pattern I correct often in lessons: a learner answers a simple everyday question with なぜなら at the start and からです at the end, and the chat suddenly sounds like a presentation.

なぜなら、時間じかんがなかったからです。 — Because I didn't have time. (formal, natural in a talk or an essay) 時間じかんがなかったんです。 — I didn't have time. (polite, the usual weight for a casual question)

What do I say when I feel defensive?

In casual speech, だって: "but, you see." It opens an excuse and sounds a little childish or defensive, so it fits friends and family, not a manager.

どうして? — Why? (casual, short) だって、知しらなかったんだもん。 — But I didn't know! (casual, defensive; もん adds a sulky feel)

Common mistakes

  • Equating polite with gentle. A polite どうして can still interrogate.
  • Attaching です to a plain verb. Say どうして行いくんですか, not どうして行いくですか.
  • Using なんでですか upward. With a client, choose どうしてですか and say what the answer is for.
  • Reading 何なんで only one way. An answer like 電車でんしゃで means the listener understood the question as "by what means"; context decides which reading is meant.
  • Answering with なぜなら every time. In short everyday exchanges it usually sounds more natural to drop it and just end with 〜からです or 〜んです.

Which "why" fits where

Word Rōmaji Register Where you hear it
なんで nande Casual Friends, family, anime, everyday speech
なんでですか nande desu ka Polite form, casual feel Friendly coworkers, light chat
どうして doushite Neutral Speech and writing, most situations
どうしてですか doushite desu ka Polite Teachers, strangers, shops
なぜ naze More formal, analytical Essays, news, talks, meetings, musing aloud
なぜですか naze desu ka Formal polite Interviews, surveys, meetings

FAQ

Is どうしても the same as どうして? No. どうしても means "no matter what," as in どうしても行いきたい, "I really want to go." With expressions of inability it means "can't, however hard I try": どうしても分わからない, "I just can't understand it." It can also express strong refusal, as in どうしても行いきたくない, "I really don't want to go."

Should I write "why" in hiragana or kanji? Hiragana is the usual choice: なんで, どうして, なぜ. The kanji 何なんで and 何故なぜ exist, 何なんで can also mean "by what means," and 何故なぜ can be read なぜ or どうして, so hiragana avoids guesswork.

Is なぜか a question? Not usually. As an adverb, なぜか means "for some reason," as in なぜか眠ねむい. Embedded in a sentence, なぜ plus か forms an embedded question meaning "why": なぜか分わかりますか, "do you know why?" The adverb gets its own article: なぜか.

How do I say "that's why"? Use だから (casual), ですから (polite) or そのため (writing) to give a result after a reason. When you suddenly understand, say あ、だからか, "ah, so that's why."

Practice

Think of your answer before opening it.

1. Ask a close friend casually why they quit the club.

Show answer

なんで部活ぶかつやめたの? — casual なんで with の, fine between close friends.

2. A coworker moved a deadline. Ask the reason politely and say you need it to plan the next task.

Show answer

次つぎの作業さぎょうの準備じゅんびのために、締め切しめきりを変かえた理由りゆうを教おしえてもらえますか。 — naming the purpose helps, but this is still an explanation request, so it does not guarantee a soft reception.

3. Which one sounds like blame: なんで言いわなかったの? or 言いわなかったのは、どうしてですか。

Show answer

Text alone can't decide. Both can sound like blame or like curiosity; relationship, voice and situation settle it, and the second is not necessarily safer.

4. In a school essay you want to ask "Why is the population decreasing?" Which "why" fits?

Show answer

なぜ人口じんこうは減へっているのでしょうか。 — なぜ sounds formal and analytical, and ~のでしょうか suits an essay question.

5. A friend asks なんで休やすんだの? You had a fever. Answer casually.

Show answer

熱ねつがあったんだ。 — the reason plus んだ is enough; no word for "because" is needed.

6. A coworker asks 昨日きのうどうして休やすんだんですか。 A learner answers なぜなら、熱ねつがあったからです。 Make it match a casual chat.

Show answer

熱ねつがあったんです。 — the original is correct but sounds like a presentation; a short 〜んです fits the chat.

7. You explain in a school presentation why you chose your topic. Is なぜなら…からです appropriate?

Show answer

Yes. — a presentation is formal and prepared, which is where なぜなら…からです belongs.

8. Your sister says どうして? after you ate her snack. Give a casual, defensive reply: you didn't know it was hers.

Show answer

だって、知しらなかったんだもん。 — だって opens an excuse and もん adds a sulky feel.

9. What does 雨あめなんで、中止ちゅうしです mean?

Show answer

Since it's raining, it's canceled. — なんで after a noun is a casual contraction of なので, not "why."

10. A coworker you are friendly with says the party has moved to Friday. Ask politely, out of simple curiosity, "Why?"

Show answer

どうしてですか。 — polite and neutral; say it with a curious voice, since tone decides how it lands.

During a trial lesson, we can role-play the same question with worried, curious and annoyed voices, then answer each one with just 〜んです.

Vocabulary from this article

Tap the block to select all of it, then copy. In Anki: File → Import, note type Basic, separator Tab — the first column becomes the front, the second the back.

なんで	なんで — why (casual)
どうして	どうして — why (neutral)
なぜ	なぜ — why (formal, analytical)
理由	りゆう — reason
遅れる	おくれる — to be late
予定	よてい — plan, schedule
熱	ねつ — fever
だって	だって — but, you see (excuse)
どうしても	どうしても — no matter what

I'm Yuuki, a Japanese teacher based in Tokyo. I've taught on italki as a Top 1% teacher and watched over 1,500 anime, so most of the example sentences on this blog come from lines I've actually heard students ask about — including this one, which comes up in nearly every beginner's first few lessons. For practice with feedback, Book a trial lesson · $13, or work through Japanese practice exercises first.

Sources: the author's own lessons and standard reference grammars.

More drills like the ones above, plus a timed JLPT-style mock test: Practice · N5 · Read something at this level: JLPT N5 stories with audio

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