When the question word is the subject, use normal order with no do: Who called you? (someone called you). When it's the object, use do / does / did: Who did you call? (you called someone).
Same situation, two choices. Pick an example.
Who called you?
The question word is the subject, so the order stays the same and there's no do.
Who did you call?
The question word is the object, so the question needs do / does / did.
| Rule | Subject question | Object question |
|---|---|---|
| The question word is | The subject | The object |
| do / does / did | No | Yes |
| Possible answer | Ana called me. | I called Ana. |
Who _____ the last piece of cake?
Paste an email or an essay. Clarigram marks the mistakes you actually make, shows the rule behind each one as a picture, and builds your practice from them.
Answer the question. If the answer replaces the subject (Ana called me), it's a subject question. If it replaces the object (I called Ana), it's an object question.