Auxilliary "do" with the subject in questions

According to English grammar I can't use the auxiliary verb do in questions if who / what /etc. stands for the subject:

– "What happened last night?" (What did happen last night?)

But what if I'm talking to somebody and he utters a sentence that already contains do to emphasize the verb? Can I leave it in my following question? Or I must reduce it (convert do verb to verb)?

– "They lied to me! Something did happen last night!"

– "What did happen last night?" (is it an error?)

Also a native speaker told me that there are situations when I can use the auxiliary do with the subject. For example:

– "Who wants to eat?"

– "Do you want to eat?" – "No."

– "Do you want to eat?" – "No, I don't."

– "Who does want to eat?" (in frustration)

Answer

You can certainly ask "What did happen last night". However, the meaning changes to one of emphasis. The question asks not only what happened, but in contrast to some earlier discussion where something else was said not to have happened.

In the exasperated "Who does want to eat", it is the same situation: emphasis.

We also need the auxiliary, if the WH-word subject is paired with the verb "do":

• What {did you do | did you*} last night.

The * marked form sounds archaic. This construction probably occurs to avoid a perceived ambiguity: "what did you last night" sounds as if "did" is still auxiliary, and the verb is missing. "What did I what last night? What did I eat? What did I watch on TV? Oh, you're asking me what I did, in some outdated way of speaking; very funny!"

(Some uses of this word order in questions still occur in British English, I think, such as, "Have you the time?" "Had you enough to eat?" Children all over the English speaking world continue to pick up the syntax from nursery rhymes: "Baa baa black sheep, have you any wool?")

Note that since the "did" is required in "what did you do yesterday", it is possible to use it with or without emphasis:

• What { did* | did } you do yesterday? [Emphasis: I already know what you didn't do; please give me the contrasting information: what you did. No emphasis: inform me about your yesterday's activities.]

We can also put the emphasis almost anywhere in the above sentence: we can emphasize "did", "you", "do" or "yesterday", in order to make any of them the focus:

In our earlier sentences, we cannot use "did" without emphasis:

• 
  

  What { did | did * } happen last night?

  
  


• 
  

  Who { does | does * } want to eat?

  
  
  

It is possible for another word to be emphasized simultaneously, in some very specific contextual situation where things are being contrasted in parallel pairs, or something of the sort:

• Okay so we established what didn't happen to Joe; so what did happen to Bob? [The focus is on Joe and Bob, and on what did happen to the former and didn't happen to the latter: two things in one sentence contrast pairwise with parallel things in the other.]