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Can someone explain it to me why I can’t use infinite-to with why, when and so on?

For example:

  • Why to use a shaver?

  • Why to use a baby carrier?

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    I'm not sure if there is an answer to why other than "because that is what the rules of English say". Or are you asking "When do we use the to-infinitive and when do we use the bare infinitive or some other form?"?
    – stangdon
    Commented Feb 10, 2018 at 13:23

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The short answer is that you cannot use it that way because the language simply does not work that way. I can see where you're coming from, though.

You can sometimes use analogies and get the right answer regarding grammar, but other times you cannot. For example, if you know that you can build prepositional phrases such as "in the house" (preposition "in" + noun phrase "the house") and "on the roof" (preposition "on" + noun phrase "the roof"), you might assume that you can also build a prepositional phrase such as "under the table" (preposition "under" + noun phrase "the table"). And you would be correct, in that case! But the assumption that you're making here, which is "If I can use one wh-word in this way, surely I can use all of them in this way," is not correct.

I would add that your examples "Why to use a shaver?" and "Why to use a baby carrier?" would be ungrammatical EVEN if you replaced the why with when or how. "How to use a baby carrier" is a phrase, not a complete sentence or question.

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This kind of speaking is called Aposiopesis

Here in this context,

  • why plus infinitive: is broken off in the beginning of the sentence. And also is called ellipses: when you leave a part of a sentence because you mentally or imaginably know what the omitted part is.

And you would hear a lot of dialogues filled with ellipses, in movies, tv show, in general life...etc, as a kind of a shortcut sentence to make the mechanism of speaking even easier and faster.

Note that a question word + infinitive cannot stand alone. We cannot say ‘what to do’. Instead we must say: ‘What shall we do?’ or ‘What is to be done?’

So your original sentence may get to be like:

  • Could/Do you know why to use a shaver?

Note that could you know isn’t essential to make the question clear. It means you can understand the point of question without the first part.

Note: you cannot use this kind of omitted words in formal writing.

an Additional Source

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    We cannot say ‘what to do’ - although it is interesting to note that "what to do" comes up as a standalone phrase sometimes in informal speech.
    – stangdon
    Commented Feb 10, 2018 at 13:22
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    The original question suggests that OP thinks "How to use a shaver?" is correct. It is not, and suffers from the same problem as "Why to use a shaver?", though this misuse is more common. "How do I use a shaver", or "Please explain how to use a shaver" would work.
    – Peter
    Commented Oct 23, 2018 at 17:34
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The simplest answer is that English requires every sentence to have at least one finite verb. Your examples only have an infinitive, so they are not sentences. They might work as part of a larger sentence with a finite verb elsewhere, but standing alone, they have no meaning.

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  • Not a great answer, I'm afraid, since we sometimes use a bare infinitive in this context ("Why use a shaver?").
    – rjpond
    Commented Feb 9, 2021 at 18:03
  • @rjpond Are you sure that’s a bare infinitive and not an inflected form missing it’s subject?
    – StephenS
    Commented Feb 9, 2021 at 19:42
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    That's a good question, but we would say "Why not be reasonable?" or "Why be unkind?" - so it appears that it's not inflected.
    – rjpond
    Commented Feb 9, 2021 at 22:19
  • @rjpond Maybe; “to be” is usually a good test, but “Why (would one) be unkind?” It’s tricky. But if you see a way to improve my answer to avoid that rabbit hole, please feel free to edit.
    – StephenS
    Commented Feb 9, 2021 at 22:32
  • You're giving a reason that the "questions" (not complete sentences) in the example are incorrect, but the user's actual question is basically "Why doesn't 'why' doesn't work in the same way as 'how' and 'when'?" Which, funnily enough, then also goes back to what you and RJPond are discussing: The other words require a "to" there, and things like "when to use a shaver" are not complete sentences/questions, while "why" does not require a "to," and "Why use a shaver?" can be a complete sentence. It just works differently than the other "wh"-words. There may not be a logic-based reason for this.
    – Linguist Lindley
    Commented Jun 21, 2021 at 1:20

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