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Do you say:

  1. What does prevent x from doing y?

OR

  1. What prevents x from doing y?

My gut tells me it's the latter, but I can't explain it. It's just a feeling. What I'd like to know is what the actual rule is, and how do I know in other cases which form is correct?

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4 Answers 4

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Your gut is right.

If your interrogative pronoun is an object, then you get inversion with a dummy auxiliary "do":

Cats chase mice → What do cats chase?

But if the pronoun is a subject, there is no inversion:

Cats chase mice → What chases mice?

Now your example has a subject interrogative pronouns

{something} prevents x from doing y → What prevents x from doing y?

(You could also ask "What does {something} prevent x from doing?", or "What does {something} prevent from doing y?", but this is not your situation)

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    Note that "What does prevent x from doing y?" can still be used, but in a specific circumstance where an earlier part of the conversation focused on what doesn't prevent it. Similar example: "Lions chase mice, right? (response: No) What about tigers? (No) Dogs? (No) Elephants (No) Well then what does chase mice?"_ In this conversation, the "does" is used to distance itself from the prior examples, i.e. we talked about things that don't chase mice, so let's now talk about things that do chase mice.
    – Flater
    Commented Nov 3, 2022 at 4:42
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    And in most such usages the "does" is explicitly styled in some fashion to indicate emphasis, which also affects pronunciation. Were you to omit the emphasis (in writing or in speech) you would usually be interpreted as incorrect rather than explicitly making that comparison.
    – Miral
    Commented Nov 3, 2022 at 5:43
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Generally speaking, you don't need auxiliary "do"-verbs in questions if the wh-word is the subject of the main verb.

In this case, "what" is the subject of "prevent", so we don't need "do".

We use the "do"-verb in subject questions when it's in the negative, or to add emphasis, especially to show contrast.

I understand that those things don't prevent x from doing y, but what does prevent x from doing y? (emphasis to show contrast)

What doesn't prevent x from doing y? (negative question)

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    In a number of dialects, usage of ‘do’ to show contrast as suggested here may also be accompanied in spoken form by additional verbal stress on the ‘does’ or ‘doesn’t’ to further emphasize the contrast, especially if the speaker is trying to be sarcastic.
    – Austin Hemmelgarn
    Commented Nov 1, 2022 at 17:00
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    @AustinHemmelgarn I'd go a step further and say the auxiliary "do" for emphasis/contrast must be stressed. Are you thinking of a counterexample, or a variety of English where the speakers don't stress this "do"?
    – gotube ♦
    Commented Nov 1, 2022 at 17:57
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    I’ve heard some usage where it was not stressed beyond what would be normal for that point in a sentence, but not much, and not recently. Most of my choice of wording above is largely just an overabundance of caution, given how many things in English have seemingly random exceptions.
    – Austin Hemmelgarn
    Commented Nov 1, 2022 at 18:06
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Case 2 is the more usual form. Case 1 could be used in very limited instances, in the form of a question, with an emphasis on does, to challenge an assumption that x is prevented from doing y.

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    or something like "Foo doesn't prevent x from doing y. What does prevent x from doing y?" - as contrast
    – Esther
    Commented Oct 31, 2022 at 20:47
  • Case 1 might also be seen outside of those instances in writing that's intended to sound antiquated (like certain fiction in a medieval setting, or perhaps very old documents). Having do in positions like that used to be optional in any context, but that hasn't been the case for centuries, so I don't recommend using it like that today.
    – Hearth
    Commented Nov 2, 2022 at 15:37
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What prevents x from doing y?

What is the subject of the question. So the word order is the same as in a statement.

Statement: It prevents x from doing y. Question: What prevents x from doing y?

What can also be the object of a sentence. In this case, an auxiliary verb (do, does, will etc.) comes before the subject.

What does he want? (what-object, he-subject)

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