So sometimes I say,

[conuly]

or hear other people say, "I don't know that something or other" and other times I say or hear "I don't know if something or other". I vaguely feel as though there is some sort of distinction here, some nuance where the two phrasings don't have exactly the same meaning, but I can't pin it down even in my speech. What does everybody else think?

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Comments

[cosmolinguist]

This might be a difference between relative clauses (which are extremely common and normal in English and very often use "that") and the subjunctive mood (which is kind of a fossil and it's not well-understood or consistently used in modern English, but it often uses "if").

The subjunctive is (supposed to be) used for hypotheticals, to express things like wishes, demands, anything we think could or should happen. So like "If I were a rich man," the reason that phrase uses "were" for a singular subject is because it's in the subjunctive. You get "if" in a lot of wishes/speculations/hypotheticals, which is why I think it might be relevant here.