I'm probably overthinking this, but I can't seem to be able to identify the function of the word "that" in the sentence "I know that...". For the sake of an example, let's use the sentence:
I know that I don't know.
Is "that" simply a conjunction here, because it connects the two clauses? If it is, it should be a subordinating conjunction. But I don't think so, because "that I don't know" is an object of the first verb "know".
My intuition tells me it's a pronoun, or a relative pronoun to be exact, but I can't explain why it is; there isn't a noun in front of the pronoun "that" for it to replace.
Is this a case in which there's an omitted noun that is supposed to be before "that"?
Any help is greatly appreciated!