答えの文で、人が最初にいますか（My brother called me）？それなら Who のすぐあとに過去形を置きます。did は使いません。
Who called?（did を使わない）
My brother called me. → Who called you?
Not "Who did call you?" — say "Who called you?". My brother は答えの文の最初にあるので、did は使いません。
やってみよう: Ira: ___ about the new rule? Amina: A teacher told me. We can't eat in class now. Ira の質問を選んでください。 (Who did tell you / Who told you / Who did you tell) 答え: Who told you — A teacher は答えの文の最初にあるので、疑問文に did は使いません：Who told you?
Who did you call?（did を使う）
答えの文で、人が動詞のあとにいますか（I called my brother）？それなら did + 動詞の原形を使います：Who did you call?
I called my brother. → Who did you call?
Not "Who you called?" — say "Who did you call?". 答えが動詞のあとにある（I called my brother）ので、did が必要です。
やってみよう: Ira: ___ at the concert? Amina: I saw a friend from my art class. Ira の質問を選んでください。 (Who saw you / Who did you help / Who helped you / Who did you see) 答え: Who did you see — Amina がその友だちを見たので、友だちは動詞のあとに来ます。だから疑問文には did を使います：Who did you see?
何があったの？：What happened?
What happened? はできごとについてたずねます。What のすぐあとに過去形を置き、did は使いません。
What happened on the bus?
Not "What did happen?" — say "What happened?". What happened? には did を使いません。
やってみよう: Ira: ___? There's paper everywhere in the hall. Amina: A box fell off the table. Ira の質問を選んでください。 (What happened / What did happen / Who called / Who did call) 答え: What happened — What happened? には did を使いません。過去形が What のすぐあとに来ます。
答えになるのはどっち？
答えの文を見ましょう。人が最初、つまり動詞の前にいますか？それなら did を使わずにたずねます。人が動詞のあとにいますか？それなら did を使ってたずねます。
The teacher helped me. → Who helped you?
I helped the teacher. → Who did you help?
やってみよう: Amina: Somebody visited me last night. Ira: Really? ___? Ira の質問を選んでください。 (Who did you call / Who called you / Who visited you / Who did you visit) 答え: Who visited you — だれかが Amina をたずねてきて、Ira はその人、つまり文の最初にいる人についてたずねています：Who visited you? did は使いません。