A noun clause is a subordinate clause that functions as a noun. Because it functions as a noun, this clause can be a subject, direct object, indirect object, object of a preposition, predicate nominative, or appositive.
I told him that he had been selected. (The noun clause 'that he had been selected' functions as a direct object.)
Where the candy bar is hidden remains a mystery. (The noun clause 'where the candy bar is hidden' functions as the subject of the sentence.)
Pay attention to what I say. (The noun clause what I say functions as the object of the preposition to.)
The news that he is alive has been confirmed. (The noun clause that he is alive is in apposition to the noun news.)
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