When to use did in a question?

Question

I have a dilemma. I know how to use this, but I'm trying to explain it to someone and I can't find a logical explanation.

So here it is:

1. Who called last night?
2. Who did you call last night?

Why does the second sentence contain "did" when the first one doesn’t?

Answer 1

Michael Swan explains this clearly in his Practical English Usage (2005.481).

When who, which, what or whose is the subject (or part of the subject), do is not normally used: Compare:

• Who phoned? (Who is the subject.)
• Who did you phone? (Who is the object.)

[...] But do can be used after a subject question word for emphasis, to insist on an answer:

• Well, tell us - what did happen when your father found you?

Answer 2

If the Wh- word represents the subject then we don't need to use do, and we don't need to change the subject and auxiliary verb.

However, It can be a bit difficult to understand if who/which is the subject. One way to find this information is to give a full answer to the question:

• X called.
• I called X.

If X is the subject of the answer, then it's usually the subject of the question, and we don't need do here. If it isn't the subject we need do:

• Who called
• Who did I call?

Hope this is helpful!