When you ask with WHO or WHAT, look at the answer. If the wh-word is the doer (subject), use no 'did' and the normal verb. If it is the receiver (object), use 'did' + base verb, like a normal question.
Quick tip: Wh-word is the doer (subject) → no 'did': Who won? Wh-word is the receiver (object) → use 'did': Who did you beat?
Subject question: no 'did'
When WHO/WHAT is the SUBJECT (the doer), do NOT use do/does/did. The verb keeps its normal past or -s form.
Who won the race? (Ethan won.)
What happened at the gym?
Who called you last night?
Who knows the answer?
Object question: use 'did'
When WHO/WHAT is the OBJECT (the receiver), use do/does/did + base verb, like a normal wh-question.
Who did you beat? (I beat Sofia.)
What did you see?
Who did Ethan call?
What does she want?
How to tell: where's the answer?
If the answer is the DOER → subject question (no 'did'). If the answer is the RECEIVER → object question (use 'did').
Who beat Sofia? → Ethan (the doer) = subject, no 'did'.
Who did Sofia beat? → Lars (the receiver) = object, use 'did'.
Who helped you? (doer) vs Who did you help? (receiver).
Same word 'who', two different structures.
Common mistakes
Not "Who did win the race?" — say "Who won the race?". 'Who' is the subject (the winner) — no 'did'.
Not "What did happen?" — say "What happened?". 'What' is the subject — no 'did'.
Not "Who you called?" — say "Who did you call?". 'Who' is the object here — you need 'did'.
Practise it
Ask: is the answer the DOER or the RECEIVER? Doer = no 'did' (Who won?). Receiver = use 'did' (Who did you beat?).