Here's how it works:
* Nouns are words that name people, places, things, or ideas.
* Noun clauses are groups of words that act like a noun, fulfilling a noun's role in a sentence.
Think of it like this: a noun clause is a whole phrase that acts as a single, big noun.
Examples:
* What you said is very interesting. (The entire phrase "what you said" acts as the subject of the sentence.)
* I know who stole the cookies. (The entire phrase "who stole the cookies" acts as the direct object of the verb "know".)
* The problem is that we are running out of time. (The entire phrase "that we are running out of time" acts as the predicate nominative, renaming the subject "problem".)
To summarize: Noun clauses don't modify; they act as the noun itself within the sentence.