Subject & Object Questions

When who, what, which or how many asks about the subject (the person or thing doing the action), there is no do, does or did. The verb keeps its normal form and tense: Who called you? (Someone called you. Who was it?)

When the question word asks about the object, use do/does/did + subject + base form: Who did you call? (You called someone. Who was it?)

• Who lives next door? – subject question, no do
• Who do you live with? – object question, with do
• What happened? – always a subject question

Don't mix the two: Who did call you? is wrong. Time words choose the tense: last night, yesterday → past; usually, when you need help → present.