人在答案的开头吗（My brother called me）？那就把过去式直接放在 Who 后面，不用 did。
Who called?（不用 did）
My brother called me. → Who called you?
Not "Who did call you?" — say "Who called you?". My brother 在答案的开头，所以不用 did。
试一试: Ira: ___ about the new rule? Amina: A teacher told me. We can't eat in class now. 选出 Ira 的问题。 (Who did tell you / Who told you / Who did you tell) 答案: Who told you — A teacher 在答案的开头，所以问句不用 did：Who told you?
Who did you call?（要用 did）
人在动词后面吗（I called my brother）？那就用 did + 动词原形：Who did you call?
I called my brother. → Who did you call?
Not "Who you called?" — say "Who did you call?". 答案在动词后面（I called my brother），所以要用 did。
试一试: Ira: ___ at the concert? Amina: I saw a friend from my art class. 选出 Ira 的问题。 (Who saw you / Who did you help / Who helped you / Who did you see) 答案: Who did you see — 是 Amina 看见了朋友，朋友在动词后面，所以问句要用 did：Who did you see?
What happened?：发生了什么？
What happened? 问的是发生了什么事。过去式直接放在 What 后面，不用 did。
What happened on the bus?
Not "What did happen?" — say "What happened?". What happened? 里没有 did。
试一试: Ira: ___? There's paper everywhere in the hall. Amina: A box fell off the table. 选出 Ira 的问题。 (What happened / What did happen / Who called / Who did call) 答案: What happened — What happened? 不用 did：过去式直接放在 What 后面。
答案在哪一边？
看答案：人在开头、在动词前面吗？那提问就不用 did。人在动词后面吗？那提问就要用 did。
The teacher helped me. → Who helped you?
I helped the teacher. → Who did you help?
试一试: Amina: Somebody visited me last night. Ira: Really? ___? 选出 Ira 的问题。 (Who did you call / Who called you / Who visited you / Who did you visit) 答案: Who visited you — 有人来看了 Amina，Ira 问的就是这个人，他在开头：Who visited you? 不用 did。