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?

  • 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 Feb 10 '18 at 13:23

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

  • 1
    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 Feb 10 '18 at 13:22
  • 1
    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 Oct 23 '18 at 17:34

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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