主語を尋ねるときは倒置しないWhen the Question Word Is the Subject
疑問文 / Questions
Who broke the window? に do は要りません。疑問詞がそのまま主語の位置にいるので、ひっくり返す必要がないからです。What happened? も同じ。見分け方は、疑問詞を人名に置き換えてみること。Tom broke the window. と自然につながれば、それは主語を尋ねる疑問文です。
"Who broke the window?" needs no do. The question word is already sitting in the subject slot, so there's nothing to flip. "What happened?" works the same way. To check, swap the wh- word for a name: if "Tom broke the window" falls out naturally, you're asking about the subject.
動かす必要がないから、動かさない
Nothing to Move, So Nothing Moves
疑問文がひっくり返るのは、知りたい部分を先頭へ引っぱり出すからです。ところが主語はもともと先頭にいます。引っぱり出す距離がゼロなので、語順はそのまま。do を呼ぶ必要もありません。Who called you? は Someone called you. の someone を who に替えただけの形です。
A question inverts because you're dragging the unknown part to the front. But the subject is already there. The distance is zero, so the word order stays put and no helper is needed. "Who called you?" is just "Someone called you" with someone swapped for who.
そのままの語順Word order untouched
Who broke theなぜ the なのか【その場で一つに決まる】話し手と聞き手のいる状況で、どれを指すか一つに決まるから the。日本語なら「（例の）」「うちの」を補って読むと感覚が近い。 window?
誰が窓を割ったの?
who が主語。do は要らず、動詞は過去形のまま。Who is the subject. No do, and the verb keeps its past form.
What happened last night?
ゆうべ何があったの?
what が主語。What did happen? とはしない。What is the subject here, so the did version isn't used.
Which team won?
どっちのチームが勝ったの?
which + 名詞も主語になれる。やはり倒置なし。Which plus a noun can be the subject too, and again nothing flips.
主語を尋ねる(倒置なし)
疑問詞が動作をする側。語順はそのままで do も使わない。The wh- word is the one doing the action. No flip, no do.
Who called you?
誰から電話があったの?
答えは「かけてきた人」。Tom called you. と続く形。The answer is the caller: "Tom called you."
目的語を尋ねる(倒置あり)
疑問詞が動作を受ける側。did を呼んでひっくり返す。The wh- word is on the receiving end, so did comes in and the order flips.
Who did you call?
誰に電話したの?
答えは「かけた相手」。You called Tom. と続く形。The answer is the person called: "You called Tom."
よくある間違い / Common Mistake
×Who did break the window?
誰が窓を割ったの?(のつもりで)
○Who broke the window?
誰が窓を割ったの?
疑問詞がすでに主語の位置にあるので、ひっくり返す相手がいません。do を入れると、通じはしますが「いったい誰が割ったんだ」という強い問い詰めに響きます。強調したいときだけ使う形なので、ふつうに尋ねるなら Who broke …? です。同じ理由で What did happen? も不自然で、What happened? が標準です。
The wh- word is already in the subject slot, so there's nothing to invert. Adding do is understandable but lands as a demand: "Who *did* break it?" reads as pressing for a culprit. That form is reserved for emphasis, so a plain question uses "Who broke …?" For the same reason "What did happen?" sounds off, and "What happened?" is the standard.
What happened?何があったの?主語を尋ねる型の代表。事故や騒ぎの場面で頻出。The flagship subject question. Turns up wherever something has gone wrong.
Who's next?次はどなた?who が主語で、be動詞が続く。列や順番の場面の定番。Who as subject with be right behind it. Standard in queues and turn-taking.
What brings you here?どうしてここに?what が主語。理由を柔らかく尋ねる言い方で、Why are you here? より当たりが弱い。What is the subject. A softer way into someone's reason than "Why are you here?"
Who says ...?誰がそんなこと言ってるの?主語の who に現在形。反論の決まり文句として使う。Subject who with a present verb, used as a set phrase for pushing back.
まとめ / Summary
疑問詞が主語なら、語順はそのまま。do は使わない。When the wh- word is the subject, nothing flips and no do appears.
見分けは、疑問詞を人名に置き換えて自然につながるか。To check, swap the wh- word for a name and see if it still reads.
Who called you? は主語、Who did you call? は目的語。"Who called you?" asks the subject; "Who did you call?" asks the object.
単独の疑問詞は三人称単数扱い。複数名詞を連れていればそちらに合わせる。A bare wh- word counts as singular; with a plural noun in tow, the noun decides.