Is "that" a relative pronoun, a conjunction, or something else in "I know that..."? [closed]

In the sentence

• He knows [that she'll be at the party].

the bracketed Complement Clause that she'll be at the party is the direct object of know
(what does he know? that she'll be at the party). Know is a verb that can take a that-complement as object.

This object complement clause is introduced by the Complementizer that; complementizers mark subordinate clauses as subordinate. There are several other complementizers besides that, and several other types of that besides complementizer that.

Complementizer that was used in Middle English to introduce any tensed clause (including adjective clauses and adverbial clauses like before that we set up the tent, still used in some dialects of English); but now it's mostly restricted to tensed complement clauses used as noun phrases.

Complementizer that eventually became the relative pronoun that as in the man that she saw; most people treat it as a relative pronoun instead of a complementizer now, but it's still a source of confusion when students are erroneously told that it's neuter.