Consider these two sentences:

Who left the door open?


Who do you want to speak to?

In the first question auxiliary do cannot normally be used, but in the latter question it can. Why is that so? What is it about the second sentence that requires auxiliary do that the first sentence doesn't have? Please include a few more examples of this pattern if possible.

  • Do you want more examples or what? Unclear what you're asking. Flagged. Commented Aug 14, 2014 at 4:14
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    @Dmitry Can you please explain what you want to know in more detail? Are you wondering about the when to use do if who is the subject, or just about do in general? Commented Aug 14, 2014 at 4:57
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    @EsotericScreenName I don't understand when I must use auxillary do, and when don't. BTW, why do you substitute my bit to few? Commented Aug 14, 2014 at 5:47
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    @Dmitry Good question. In general (there are a few exceptions), bit is for uncountable nouns and few is for countable ones. You can count the number of examples, so I changed it to few. For example, you would say a bit of money, running, water, or glue. Commented Aug 14, 2014 at 6:08
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    Off the top of my head I don't know the answer to this one, but I can't see why it's "Unclear". It is of course possible to ask "Who did leave the door open?" in certain contexts, but I think probably the reason why do doesn't normally work there is something to do with whether who is the subject or the object in syntactic terms. Commented Aug 14, 2014 at 20:54

1 Answer 1


These are called subject questions (as your first example) and object questions (as your second). Here we are talking about "do", so this is present simple and past simple tense. Let us take one example:

Paul wants to speak to him.

1) If we want to know who "him" is, "him" is the object and the question we will ask is an object question. There will be an auxiliary verb, which will come between the question word and the subject:

Who does Paul want to speak to?

2) If we ask a question about "Paul" (the subject), however, it will be a subject question, and then all we need to do is substitute the subject with a question word:

Who wants to speak to him?

Further reading with examples: in this grammar book

3) It is possible to add an auxiliary verb in subject questions, that has already been discussed here. To sum it up, it adds emphasis, as in:

A: Who wants to speak to him? Jack?
B: No.
A: Jim?
B: No.
A: Well, then who **does** want to speak to him?

More examples of that can be found in the link.

  • what about of : 1.can i change it ? vs do i can change it ? 2. does it would change ? vs would it change? What is the correct form? Commented Mar 29, 2016 at 22:50
  • @GilbertoIbarra The shortest answer I can give you: Modal verbs (can, would) do not take an auxiliary verb. They form the question themselves, just like 'to be'. So it is: "Can I change it?" and "Would it change?"
    – fluffy
    Commented Apr 4, 2016 at 9:45

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