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Below are three examples:

  • That's all I know to do.

  • That's all I know how to do.

  • That's all I know what to do.

Which of the three is the most idiomatic way of expressing the idea that what was mentioned was all that I can do, since I don't know how to do anything else. To me, 'how to do' sounds the most natural but lacks logic, since it's about WHAT I can do and not HOW I do them. Also, as much as I am tempted to go with 'All I know to do', isn't the verb 'know' usually not followed by infinitives?

Thanks in advance.

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That's all I know about it.

would be the general idiomatic expression to say you have a limited knowledge about the subject.

That's all I know (about) how to do (it).

would mean you have limited knowledge on "how" to do something

That's all I know (about) what to do.

would mean you have limited knowledge on "what" needs to be done in a situation.

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  • You added "about" where the poster made no mention of the word. that changes the meaning somewhat. I'm not sure that's what they were asking.
    – Curtis White
    Commented Jan 26, 2017 at 2:28
  • That's right. Thanks for the input Peter but I am looking at an expression that relates to what someone knows to do, not what they know about a certain subject.
    – JUNCINATOR
    Commented Jan 26, 2017 at 2:47
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All three are okay, but they are used in different contexts.

That's all I know to do

This one is for when you are speaking about your knowledge of which options you have. As in, "I can't think of anything else to try."

That's all I know how to do

This is for speaking about your knowledge of certain actions. As in, "I don't know how to do anything else. Therefore, there's nothing more I can try."

That's all I know what to do

This is is pretty much the same as the first, but it's not used this way in standard English. It is chiefly a variant of certain dialects within England, and frankly is probably not very common there either. Since I'm Canadian, I can't speak much to English dialects outside my area. Perhaps an English user could give a clearer answer to that.

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All the sentences are grammatical.

That's all I know to do/That's all I know what to do.

Both the sentences have the same meaning, that is, I don't know there's more to do than I told you.

That's all I know how to do.

That's a different meaning. It means "I don't know more than I told you how to do".

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  • I can't pinpoint the rule that would make it ungrammatical, but "that's all I know what to do" sounds very unnatural and would at least get a raised eyebrow from me if somebody said it.
    – Emmabee
    Commented Jan 26, 2017 at 4:44
  • @Emmabee, I am sorry. It's a typo.
    – Khan
    Commented Jan 26, 2017 at 8:16

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